Google Maps -> GeoJSON conversion

* mounted functions in the google.maps.geojson namespace (`google.maps.geojson.from()` instead of `new GeoJSON()`)
* created `google.maps.geojson.to()`
* removed duplicate points in GeoJSON polygons for closing rings when converting to GMaps overlays; fixes #17
* setup a test suite using QUnit which tests GeoJSON -> GMaps, GMaps -> GeoJSON, and GeoJSON -> GMaps -> GeoJSON

See #20 for an explanation of why `google.maps.geojson.to()` doesn't produce GeometryCollections.

This is a pretty significant change so I understand if you have any concerns about merging it.