Support complex polygon in Geofence class

This commit adds support for "interior rings" aka "holes" inside
polygons as per the GeoJSON spec:

  http://www.geojson.org/geojson-spec.html#polygon

See discussion thread here:

  http://lists.geojson.org/pipermail/geojson-geojson.org/2013-November/000863.html

Maniphest task: https://code.uber.com/T1955
Merge pull request #7 from uber/as-complex-polygon

Support complex polygons for geofences