First pass at MBTiles for MBXMapKit

This worked for some very simple testing on the iOS side. I don't have a Mac Dev program membership to see what happens on the Mac side. I've got things #ifdef'ed pretty well, so I'm optimistic this could be merged without breaking anything for people who haven't turned on MBTiles support, but this could definitely use some further work.

I'm interested in polishing up to the point that it's ready for production, and I think I will need help (code review & testing) in order to do that.

This is for issue #3, https://github.com/mapbox/mbxmapkit/issues/3