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

Header file and API tweaks to fix for #83, #91, and #93:
 * Re-introduces `MBXMapKit.h` so view controllers only need one include (see #93) 
 * Moves enum typedefs and other constants to `MBXConstantsAndTypes.h` so they can be included in header files which need them without causing import loops (necessary to implement #93)
 * Adds a public category on MKMapView for `setCenterCoordinate:zoomLevel:animated:` (see #83)
 * Aligns arguments for  the `-[MBXOfflineMapDownloader beginDownloadingMapID...]` methods (see #91)
 * Moves `+qualityExtensionForImageQuality:` and `+markerIconURLForSize:` to a private API (see #91)
 * Gets rid of MBXError and replaces it with a private category on NSError (see #91)