fs: Break fs package to top-level and introduce ObjectAPI interface.

ObjectAPI interface brings in changes needed for XL ObjectAPI layer.

The new interface for any ObjectAPI layer is as below

```
// ObjectAPI interface.
type ObjectAPI interface {
        // Bucket resource API.
        DeleteBucket(bucket string) *probe.Error
        ListBuckets() ([]BucketInfo, *probe.Error)
        MakeBucket(bucket string) *probe.Error
        GetBucketInfo(bucket string) (BucketInfo, *probe.Error)

        // Bucket query API.
        ListObjects(bucket, prefix, marker, delimiter string, maxKeys int) (ListObjectsResult, *probe.Error)
        ListMultipartUploads(bucket string, resources BucketMultipartResourcesMetadata) (BucketMultipartResourcesMetadata, *probe.Error)

        // Object resource API.
        GetObject(bucket, object string, startOffset int64) (io.ReadCloser, *probe.Error)
        GetObjectInfo(bucket, object string) (ObjectInfo, *probe.Error)
        PutObject(bucket string, object string, size int64, data io.Reader, metadata map[string]string) (ObjectInfo, *probe.Error)
        DeleteObject(bucket, object string) *probe.Error

        // Object query API.
        NewMultipartUpload(bucket, object string) (string, *probe.Error)
        PutObjectPart(bucket, object, uploadID string, partID int, size int64, data io.Reader, md5Hex string) (string, *probe.Error)
        ListObjectParts(bucket, object string, resources ObjectResourcesMetadata) (ObjectResourcesMetadata, *probe.Error)
        CompleteMultipartUpload(bucket string, object string, uploadID string, parts []CompletePart) (ObjectInfo, *probe.Error)
        AbortMultipartUpload(bucket, object, uploadID string) *probe.Error
}
```

fs-utils_test.go

@@ -0,0 +1,114 @@
+/*
+ * Minio Cloud Storage, (C) 2016 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package main
+
+import (
+	"testing"
+)
+
+//Validating bucket name.
+func ensureBucketName(t *testing.T, name string, testNum int, pass bool) {
+	isValidBucketName := IsValidBucketName(name)
+	if pass && !isValidBucketName {
+		t.Errorf("Test case %d: Expected \"%s\" to be a valid bucket name", testNum, name)
+	}
+	if !pass && isValidBucketName {
+		t.Errorf("Test case %d: Expected bucket name \"%s\" to be invalid", testNum, name)
+	}
+}
+
+func TestIsValidBucketName(t *testing.T) {
+	testCases := []struct {
+		bucketName string
+		shouldPass bool
+	}{
+		//cases which should pass the test
+		//passing in valid bucket names
+		{"lol", true},
+		{"1-this-is-valid", true},
+		{"1-this-too-is-valid-1", true},
+		{"this.works.too.1", true},
+		{"1234567", true},
+		{"123", true},
+		{"s3-eu-west-1.amazonaws.com", true},
+		{"ideas-are-more-powerful-than-guns", true},
+		{"testbucket", true},
+		{"1bucket", true},
+		{"bucket1", true},
+		//cases for which test should fail
+		//passing invalid bucket names
+		{"------", false},
+		{"$this-is-not-valid-too", false},
+		{"contains-$-dollar", false},
+		{"contains-^-carrot", false},
+		{"contains-$-dollar", false},
+		{"contains-$-dollar", false},
+		{"......", false},
+		{"", false},
+		{"a", false},
+		{"ab", false},
+		{".starts-with-a-dot", false},
+		{"ends-with-a-dot.", false},
+		{"ends-with-a-dash-", false},
+		{"-starts-with-a-dash", false},
+		{"THIS-BEINGS-WITH-UPPERCASe", false},
+		{"tHIS-ENDS-WITH-UPPERCASE", false},
+		{"ThisBeginsAndEndsWithUpperCase", false},
+		{"una ñina", false},
+		{"lalalallalallalalalallalallalala-theString-size-is-greater-than-64", false},
+	}
+
+	for i, testCase := range testCases {
+		ensureBucketName(t, testCase.bucketName, i+1, testCase.shouldPass)
+	}
+}
+
+//Test for validating object name.
+func ensureObjectName(t *testing.T, name string, testNum int, pass bool) {
+	isValidObjectName := IsValidObjectName(name)
+	if pass && !isValidObjectName {
+		t.Errorf("Test case %d: Expected \"%s\" to be a valid object name", testNum, name)
+	}
+	if !pass && isValidObjectName {
+		t.Errorf("Test case %d: Expected object name \"%s\" to be invalid", testNum, name)
+	}
+
+}
+
+func TestIsValidObjectName(t *testing.T) {
+	testCases := []struct {
+		objectName string
+		shouldPass bool
+	}{
+		//cases which should pass the test
+		//passing in valid object name
+		{"object", true},
+		{"The Shining Script <v1>.pdf", true},
+		{"Cost Benefit Analysis (2009-2010).pptx", true},
+		{"117Gn8rfHL2ACARPAhaFd0AGzic9pUbIA/5OCn5A", true},
+		{"SHØRT", true},
+		{"There are far too many object names, and far too few bucket names!", true},
+		//cases for which test should fail
+		//passing invalid object names
+		{"", false},
+		{string([]byte{0xff, 0xfe, 0xfd}), false},
+	}
+
+	for i, testCase := range testCases {
+		ensureObjectName(t, testCase.objectName, i+1, testCase.shouldPass)
+	}
+}