Place model support and geo API mods

Place model support and geo API mods

api.py:

* Adding  API.geo_similar_places (geo/similar_places.json).
* Updating geo_search, geo_id, nearby_places, and reverse_geocode to return Place model instances as appropriate, rather than raw query result JSON data.  The geo APIs parse the 'result' element into a single Place or a list of Place instances.

models.py:

* Status: Now uses Place model if place attribute present in tweet/status body.
* Adding BoundingBox, Place model classes.

NOTE: Breaking Changes

* Clients referencing Status.place or existing geo  functions (see list above) must be reviewed and updated to use proper attribute access.  The geo methods now return Place model instances rather than the raw JSON query data.