Lay foundation for implementing MBTiles databases
changed init method name
further implemented mbtiles feature (incomplete)
further implemented MBTiles datasource and overlay
finished implementing first crude version of mbtiles support
added mbtiles test file
Added some error checking
Removed MBXMBTilesDatabase class

MBTiles functionality is now all implemented in MBXMBTilesOverlay.
Removed MBXMBTilesDatabase class

MBTiles functionality is now all implemented in MBXMBTilesOverlay.
Merge branch 'issue3-mbtiles' of https://github.com/tkrajacic/mbxmapkit into issue3-mbtiles

Conflicts:
	MBXMapKit/MBXMBTilesOverlay.m
minor code cleanup and beautification
fixed and enhanced overzoom
MBTiles files are now read asynchronously
Implemented unified sqlite connection per overlay
Viewcontroller of test app correctly deallocs overlays after mapview disappears
Added two mbtiles files for testing (Courtesy of @wsnook)
Renamed mbtiles metadata key constants
Merge pull request #110 from tkrajacic/issue3-mbtiles

Merging this into mapbox/mbxmapkit issue3-mbtiles to try it out.