Implement proper delimiter and prefix handling

With this change Minio server now responds with, delimited
'object names' in conjunction with prefix filtering

~~~
<ListBucketResult>
  <Name>example-bucket</Name>
  <Prefix></Prefix>
  <Marker></Marker>
  <MaxKeys>1000</MaxKeys>
  <Delimiter>/</Delimiter>
  <IsTruncated>false</IsTruncated>
  <Contents>
    <Key>sample.html</Key>
    <LastModified>2011-02-26T01:56:20.000Z</LastModified>
    <ETag>example-bucket#sample.html</ETag>
    <Size>142863</Size>
    <Owner>
      <ID>minio</ID>
      <DisplayName>minio</DisplayName>
    </Owner>
    <StorageClass>STANDARD</StorageClass>
  </Contents>
  <CommonPrefixes>
    <Prefix>photos/</Prefix>
  </CommonPrefixes>
</ListBucketResult>
~~~

~~~
<ListBucketResult>
  <Name>example-bucket</Name>
  <Prefix>photos/2006/</Prefix>
  <Marker></Marker>
  <MaxKeys>1000</MaxKeys>
  <Delimiter>/</Delimiter>
  <IsTruncated>false</IsTruncated>

  <CommonPrefixes>
    <Prefix>photos/2006/feb/</Prefix>
  </CommonPrefixes>
  <CommonPrefixes>
    <Prefix>photos/2006/jan/</Prefix>
  </CommonPrefixes>
</ListBucketResult>
~~~

pkg/storage/storage.go

@@ -43,12 +43,8 @@ type BucketMetadata struct {
 }
 
 type ObjectMetadata struct {
-	Bucket    string
-	Key       string
-	Maxkeys   int
-	Prefix    string
-	Marker    string
-	Delimiter string
+	Bucket string
+	Key    string
 
 	ContentType string
 	Created     time.Time