Udpated for latest GMS SDK 1.3, plus refactored dragging code

Hi Robert,

I made a few updates to the project as follows:

1) Updated for Google Maps for iOS SDK v 1.3
2) Changed the YOUR_API_KEY in AppDelegate to look for a named constant (kPrivateMapsAPIKey), which can be defined in a separate file (GMPrivateMapsKey.m) - the main reason for this is avoid accidentally committing a private key to a public repo; the .gitignore file is updated to include the private API key file
3) Refactored all the drag related code into a separate helper class GMDraggableMarkerManager to help keep the view controller clean
4) Updated the view controller to implement GMDraggableMarkerManagerDelegate to enable it to receive drag related events (onMarkerDrag, onMarkerDragStart, onMarkerDragEnd)

Regards,

Stephen