api: Implement multiple objects Delete api - fixes #956

This API takes input XML input in following form.

```
<?xml version="1.0" encoding="UTF-8"?>
<Delete>
    <Quiet>true</Quiet>
    <Object>
         <Key>Key</Key>
    </Object>
    <Object>
         <Key>Key</Key>
    </Object>
    ...
</Delete>
```

and responds the list of successful deletes, list of errors
for all the deleted objects.

```
<?xml version="1.0" encoding="UTF-8"?>
<DeleteResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
  <Deleted>
    <Key>sample1.txt</Key>
  </Deleted>
  <Error>
    <Key>sample2.txt</Key>
    <Code>AccessDenied</Code>
    <Message>Access Denied</Message>
  </Error>
</DeleteResult>
```

pkg/fs/fs-multipart.go

@@ -202,7 +202,7 @@ func saveParts(partPathPrefix string, mw io.Writer, parts []CompletePart) *probe
 			if !os.IsNotExist(e) {
 				return probe.NewError(e)
 			}
-			// Some clients do not set Content-MD5, so we would have
+			// Some clients do not set Content-Md5, so we would have
 			// created part files without 'ETag' in them.
 			partFile, e = os.OpenFile(partPathPrefix+fmt.Sprintf("$%d-$multiparts", part.PartNumber), os.O_RDONLY, 0600)
 			if e != nil {