Add district location search by zip and lat / long

Adds coverage for the districts section of the sunlight-congress API
Tests for district location search
Add search for legislators by latitude and longitude
Test for legislator search by lat / long
Merge pull request #2 from rjsamson/districts

Add districts location search
Merge pull request #3 from rjsamson/latlong

Add seach by latitude and longitude to legislators search
merging to upstream