automatically cache/use TileJSON

[incanus]

We should cache TileJSON downloaded as part of RMMapboxSource.

1. Save download to disk.
2. Add argument to initializers to force cache clear & re-download.
3. Automatically find cached TileJSON according to passed mapID and use instead of hitting network.
4. Also support auto-finding TileJSON named according to mapID in the app bundle and use instead of hitting network.

These will allow -initWithMapID: to be used regardless of network connectivity instead of differing in behavior from a manually passed TileJSON object.