Adds Render(), JSON() and XML() facilities.

[manucorporat]

Adds Encoder.Render() facility. Since encoder.Must is very frequently used:

enc.Render(result) == encoder.Must(enc.Encode(result))

and now you can use:

m.Use(encoder.JSON())

instead of:

m.Use(func(c martini.Context, w http.ResponseWriter) {
     c.MapTo(encoder.JsonEncoder{}, (*encoder.Encoder)(nil))
     w.Header().Set("Content-Type", "application/json; charset=utf-8")
})