[WIP] Tiled OSM data

Makes requests for OSM data to a highly-cacheable resource instead of the API map call.

A couple sticking points;
- When a change is saved, iD clears and re-requests all existing data. Chances are the tiles will not have expired by the time iD requests the tiles again (because the minutely diff hasn't been processed yet -- most systems using the minutely diffs are ~1.2 minutes behind at best).
- The goal is to have the cache be served entirely from a static store (like S3), making it extremely fault tolerant, but there's an awful lot of tiles that would need to be pre-generated for that to make sense, so right now the endpoint uses TileStache to cache requests as they come in.