Added input-group support

```
{{forms.email_field(form.email, prefix='@')}}
{{forms.text_field(form.phone, prefix='+30')}}
{{forms.text_field(form.money, prefix='$', suffix='.00')}}
```
![screen shot 2014-09-22 at 23 29 06](https://cloud.githubusercontent.com/assets/125676/4364408/c86aab04-429f-11e4-8830-f587d95298cd.png)