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
improved README + code quality badges
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
Update README.rst

Fixed ReST text
updated VERSION to 0.2 final
Add InBBOXFilter detail to Readme.
Update README.rst
Merge pull request #19 from alexdebrie/master

Add InBBOXFilter detail to Readme
unit test for issue #21