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.

README.md

@@ -40,7 +40,7 @@ Gin is a web framework written in Go (Golang). It features a martini-like API wi
     - [Bind Query String or Post Data](#bind-query-string-or-post-data)
     - [Bind HTML checkboxes](#bind-html-checkboxes)
     - [Multipart/Urlencoded binding](#multiparturlencoded-binding)
-    - [XML, JSON and YAML rendering](#xml-json-and-yaml-rendering)
+    - [XML, JSON, YAML and ProtoBuf rendering](#xml-json-yaml-and-protobuf-rendering)
     - [JSONP rendering](#jsonp)
     - [Serving static files](#serving-static-files)
     - [Serving data from reader](#serving-data-from-reader)
@@ -871,7 +871,7 @@ Test it with:
 $ curl -v --form user=user --form password=password http://localhost:8080/login
 ```
 
-### XML, JSON and YAML rendering
+### XML, JSON, YAML and ProtoBuf rendering
 
 ```go
 func main() {
@@ -905,6 +905,19 @@ func main() {
 		c.YAML(http.StatusOK, gin.H{"message": "hey", "status": http.StatusOK})
 	})
 
+	r.GET("/someProtoBuf", func(c *gin.Context) {
+		reps := []int64{int64(1), int64(2)}
+		label := "test"
+		// The specific definition of protobuf is written in the testdata/protoexample file.
+		data := &protoexample.Test{
+			Label: &label,
+			Reps:  reps,
+		}
+		// Note that data becomes binary data in the response
+		// Will output protoexample.Test protobuf serialized data
+		c.ProtoBuf(http.StatusOK, data)
+	})
+
 	// Listen and serve on 0.0.0.0:8080
 	r.Run(":8080")
 }