Add our field when any subclass of GIS.

We now add our serializer GeometryField whenever we encounter a subclass
of the base django GeometryField, rather than check on per-class basis.
This allows our serializer to be used on custom GeometryField
subclasses.

All tests pass.
Merge pull request #16 from philipn/better_geometryfield_detection

Add our field when any subclass of GIS.