Fix for #7 (toggle between overlay and Apple map)

This exposes MBXMapViewTileOverlay in MBXMapKit.h so that it becomes possible to "toggle" layers by using MKMapView's `addOverlay:` and `removeOverlay:` methods.

The sample code in MBXViewController.m demonstrates how to accomplish the toggling effect. It uses a timer to swap the Mabox map layer in and out at 5 second intervals. It also introduces an extensible mechanism (`switch(whichDemo){...}`) for easily selecting which feature should be demonstrated at load time.