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

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")
})
```