Add sourceInfo to NotificationEvent (#3937)

This change adds information like host, port and user-agent of the
client whose request triggered an event notification.

E.g, if someone uploads an object to a bucket using mc. If notifications
were configured on that bucket, the host, port and user-agent of mc
would be sent as part of event notification data.

Sample output:
```
"source": {
          "host": "127.0.0.1",
          "port": "55808",
          "userAgent": "Minio (linux; amd64) minio-go/2.0.4 mc ..."
}
```

cmd/bucket-notification-datatypes.go

@@ -176,6 +176,14 @@ const (
 	eventVersion = "2.0"
 )
 
+// sourceInfo represents information on the client that triggered the
+// event notification.
+type sourceInfo struct {
+	Host      string `json:"host"`
+	Port      string `json:"port"`
+	UserAgent string `json:"userAgent"`
+}
+
 // NotificationEvent represents an Amazon an S3 bucket notification event.
 type NotificationEvent struct {
 	EventVersion      string            `json:"eventVersion"`
@@ -187,6 +195,7 @@ type NotificationEvent struct {
 	RequestParameters map[string]string `json:"requestParameters"`
 	ResponseElements  map[string]string `json:"responseElements"`
 	S3                eventMeta         `json:"s3"`
+	Source            sourceInfo        `json:"source"`
 }
 
 // Represents the minio sqs type and account id's.