Add districts location search

This commit adds coverage for the district location search portion of the sunlight congress API. Districts can be searched for by either zip code or latitude / longitude like so:

```ruby
Sunlight::Congress::District.by_zipcode(12186)
=> #<Sunlight::Congress::District:0x007fc9029e5340 @state="NY", @district=20>
Sunlight::Congress::District.by_latlong(42.6525, -73.7567)
=> #<Sunlight::Congress::District:0x007fc9029f7978 @state="NY", @district=20>
```