Restructure and enable South

This PR restructures the package to follow the usual Django conventions.

1. It changes the top-level namespace from `django_livefield` to `livefield`, which requires a major version bump.
2. It restructures the internal file layout.
3. It pushes all GIS-related functionality into the `livefield.gis` submodule. This allows users who don't need GIS support to import `LiveField` and `LiveModel` without installing the [geospatial libraries](https://docs.djangoproject.com/en/1.6/ref/contrib/gis/install/geolibs/).
4. It enables South introspection for `LiveField`.

@dyve, this should fix hearsaycorp/django-livefield#11.