Adds Encoder.Render() facility.

Since encoder.Must is very frequently.
enc.Render(result)  ==  encoder.Must(enc.Encode(result))
Adds encoder.JSON() and encoder.XML() facilities.

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")
})
```