Rough geojson controller - review of this approach

To do:
* [x] Allow returning only a specific set of attributes
* [ ] Add flag to not return full post object as property
* [ ] Add a way to filter posts on geom bbox on only 1 attribute 
* [ ] GeoJSON from revision ?
* [ ]  Add some way to flag an attribute as the primary location source.


**Can you review this and the ideas here on #72** :

* Add an option for geojson data format http://www.geojson.org/
  * This should also include an optional filter based on attribute ie. only returning geojson points/geometries from a particular attribute
  * Default to returning all geodata we have (this may mean a post is visible multiple times)
* Support returning geojson for
  * /posts
  * /posts/:id
  * /posts/:id/updates
  * /posts/:id/revisions ? (could be useful for location over time?)
  * /sets/:id/posts
* We do NOT need to  return geojson for translations. Location shouldn't change between translations
* Add some way to flag an attribute as the primary location source.
  * This would become the default attribute returned in GeoJSON
* GeoJSON returned should match the simplestyle spec https://github.com/mapbox/simplestyle-spec
* GeoJSON returned should be able to be bounding boxed for loading in a tiled manner.
** Its possible that tiled geojson should come via urls similar to png tiles ie. /posts.geojson/{z}/{x}/{y} (zoom/x/y)
http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames