gin already support http2, while previously not support server push.
Add Pusher() function to extend the ResponseWriter interface.
```golang
// get http.Pusher
if pusher := c.Writer.Pusher(); pusher != nil {
// use pusher.Push() to do server push
}
```
<img width="881" alt="screen shot 2018-03-07 at 11 20 49 pm" src="https://user-images.githubusercontent.com/16014993/37100619-680c00c6-225e-11e8-9352-76ec3bd62894.png">
