Create ability for conduit to handle django.contrib.gis.db.models.GeometryField(s)

I have Travis CI pointing to [this dev branch and it is passing](https://travis-ci.org/thebigspoon/django-conduit/builds/40906451).

Why only the GeometryField instead of [all geometry field types](https://docs.djangoproject.com/en/dev/ref/contrib/gis/model-api/)?

GeometryField is the base type for [all other geometry types](https://github.com/django/django/blob/master/django/contrib/gis/db/models/fields.py). The added value for these subclasses is validation and type checking. When saving the geometry to the model, Django will check to [make sure you are persisting a geometry of the same type to the database](https://github.com/django/django/blob/master/django/contrib/gis/db/models/proxy.py). GeometryField will not persist an invalid or broken geometry regardless of the type, and this seems like the most important validation. For geometry comparison type checks, GEOS can compare the raw geometry to figure out what types we are working with if anyone needs that ability. I can see an argument for adding the other geometry type fields based on validation reasons. But adding them will be easy in the future. They are just a couple lines in `_from_basci_type` and `_to_basic_type`.