cocoapods support, mbtiles enhancements, bug fixes

The Podfile (and Podfile.lock) are the only items needed to use CocoaPods to build WhirlyGlobe.
Add support for loading an MBTiles file from the filesystem
Fix bug with 'northUp' always being true
Add setHeading:position: to WhirlyGlobeViewController
Add code to set the drawPriority of the tileLoader so multiple layers don't z-fight.