Add age as search parameter

## To test

```py
from famille.models.users import Prestataire
from datetime import date
f = Prestataire.objects.first()
f.birthday= date(1989, 4, 3)
f.save()
```

## To do
- add birthday field to fixtures
- use radio button (?)
![](http://dl.dropbox.com/u/22473587/Screenshots/sejv.jpg)
- unit tests (?)