Fix NSInternalInconsistencyException related to KVO
Fix NSInternalInconsistencyException related to KVO

When the map view is deallocated before the delegate proxy, the proxy can't remove itself from the map view's KVO observers and an `NSInternalInconsistencyException` was thrown ("An instance … of class MKMapView was deallocated while key value observers were still registered with it. Current observation"). The issue was reproducible on macOS 10.12.6/Xcode 9.2 by running the unit test `testDeallocMapView`.

To fix this problem, the proxy now references the map view strongly so that the removal can happen in its `dealloc` method. Fixes #136

choefele/CCHMapClusterController,choefele/CCHMapClusterController