Implements geospatial filter support.

You can now define a `GeometryFilter` on FilterSet and use any of the
provided lookup types to do geospatial lookups.

The usage of LOOKUP_TYPES in __new__ is for compatability with
https://github.com/philipn/django-rest-framework-chain and
`AllLookupsFilter`
+ Documentation
Add filterset.py
require django-filter + geofilter test
Merge remote-tracking branch 'upstream/master' into geofilter_support
improved tests
make django-filter dependency optional
change VERSION to 0.2 beta
converted README to Restructured Text