Add leaflet form widget and admin

![capture du 2013-08-29 12 19 03](https://f.cloud.github.com/assets/546692/1048836/78b6ad94-1094-11e3-86d8-c3e88626a31d.png)

Usage is very easy ( **Requires Django 1.6** ) : 

```python
# admin.py

from django.contrib import admin
from leaflet.admin import LeafletGeoAdmin

from .models import Region


admin.site.register(Region, LeafletGeoAdmin)
```

Or in form : 

```python
    from django import forms
    from leaflet.forms.fields import PointField
    from .models import WeatherStation

    class WeatherStationForm(forms.ModelForm):
        geom = PointField()

        class Meta:
            model = WeatherStation
            fields = ('name', 'geom')

```


**TODO**

* [ ] Check bidirectional text
* [x] Translate JS texts
* [x] Common form field (outside admin)
* [x] Prevent multiple geometries if field type is not multi
* [x] Bypass Wicket for Multi-geometry types
* [x] Fix CSS display of wkt in debug mode
* [x] Django <1.6 compat using Floppyforms ?
* [ ] Option to remove edit/delete buttons
* [x] Write template tests 
* [ ] Write some JS tests
* [x] Generic geometry tests
* [x] Generic geometry collection support