Initial commit of geocoding functionality

Geocode a Series or list of strings and get back a GeoDataFrame with the
Point objects and a full geocoded address.
Change assertIn to assert_ for python 2.6 support
Catch geocoding erros and fill with empty rows

Catch GeocoderResultError and ValueError. Fill the resulting GeoDataFrame
with an empty row.
Add some more documentation about geopy
Merge pull request #39 from jwass/geocode

Initial commit of geocoding functionality