Restructure to match Django convention

This PR restructures the project enough to warrant a major version bump.

1. It renames the top-level namespace to `livefield` (instead of `django_livefield`).
2. It reshuffles files internally to match the usual Django layout.
3. It pushes all GIS-related functionality into the `livefield.gis` namespace, which allows users who don't need GeoDjango support to install this app without the geospatial libraries.

@dyve, this fixes #11.