Add filtering by bounding box to asset map

[benjwadams]

From #19. Add bounding box filtering capabilities to asset map.

[benjwadams]

If we want to use Mongo's 2dsphere index to do this sort of thing, we have to be much more careful about what is stored inside GeoJSON coordinates fields.

[lukecampbell]

This will have to wait until we have PostGIS, I really don't want to get the code dirty with MongoDB's geospatial capabilities when we're planning on using PostGIS in the near future.

#178