#83 Getting started with MBXMapKit.h
#83 MBXError is now  NSError (MBXError)
#83 #91 Private API for url helper methods
#83 #91 Align beginDownloadingMapID method args
#83 Category method for set center/zoom
adjust getter per Apple conventions
Merge pull request #97 from mapbox/issue83

Re-introduce MBXMapKit.h + misc API tweaks. Fixes #83, #91, and #93