Let legislators_locate and districts_locate find by address

There was an issue with geocoding on the sunlight api wrapper (https://github.com/sunlightlabs/ruby-sunlight/issues/7) and konklole suggested I added similiar functionality to this gem. The PR lets legislators_locate and districts_locate find by address and uses the geocoder gem to do so. 

Let me know if there is anything I can to get this PR through.