Add support for Protobuf format response and unit test (#1479)

`Gin` now have the `protobufBinding` function to check the request format, but didn't have a protobuf response function like `c.YAML()`.
In our company [ByteDance](http://bytedance.com/), the largest internet company using golang in China, we use `gin` to transfer __Protobuf__  instead of __Json__, we have to write some internal library to make some wrappers to achieve that, and the code is not elegant. So we really want such a feature.

render/render.go

@@ -27,6 +27,7 @@ var (
 	_ Render     = MsgPack{}
 	_ Render     = Reader{}
 	_ Render     = AsciiJSON{}
+	_ Render     = ProtoBuf{}
 )
 
 func writeContentType(w http.ResponseWriter, value []string) {