create Cocoa umbrella header

[incanus]

This isn't critical for b1, but should be a trivial lift. We should have a combined header much like MapKit now that we have not just MGLMapView.h but also MGLAnnotation.h for the annotation protocol (plus more in future). We should just include everything the developer needs in an MapboxGL.h.

Static library users will:

#import "MapboxGL.h"

Dynamic framework (#828) and CocoaPods users will:

#import <MapboxGL/MapboxGL.h>

Or:

import MapboxGL