Limit memory mode at 100 buckets and return error appropriately

~~~
mc: <ERROR> Failed to create bucket for URL [http://localhost:9000/newbucket-101]. Reason: [You have attempted to create more buckets than allowed.].
~~~

Response from server

pkg/api/api_object_handlers.go

@@ -163,6 +163,10 @@ func (server *minioAPI) putObjectHandler(w http.ResponseWriter, req *http.Reques
 		w.WriteHeader(http.StatusOK)
 	case drivers.ObjectExists:
 		{
+			// we need to debate about this, if this is the right message to send back
+			// https://github.com/minio-io/minio/issues/505
+
+			// Ideally we can use 405 Method No Allowed
 			writeErrorResponse(w, req, NotImplemented, acceptsContentType, req.URL.Path)
 		}
 	case drivers.BadDigest: