A start towards python 3 support

This is a start at Python 3 support, fixing some of the obvious things.  I can get some simple `GeoSeries` objects defined, but much isn't yet working, including tests.

Introduces a dependency on `six`.  It would be nice to use `pandas.compat` instead to avoid a new dependency, but I don't think we can if we want to keep pandas 0.12 support (@jtratner please correct me if I'm wrong about that).