tox, support for input groups in the widgets

* tox : so we don't have to wait for travis :smile: 
* input groups in forms : add support from python:

In the templates, it was possible to do 
```
{% bootstrap_field form.field addon_before="some text" %}
```

It is now also possible to do :
```python
class MyForm(forms.Form):
     my_field = forms.CharField(widget=forms.TextInput(attrs={'addon_before': 'some text'}))
```

**TODO** document it, because it is not ..
On which page should I do that ?