Animation for draggable markers

Hi Robert,

A few more updates I was wondering if you might have a look at:

- Markers now animate smoothly upwards at the start of a drag, and do an animated "jump" at the end of drag (done by temporarily replacing the marker with a UIImageView)
- The hit test for whether a marker was touched now uses the rect for the marker icon instead of a fixed rect size
- Markers are now optionally draggable; add the markers you want to make draggable to the GMDraggableMarkerManager instance
- The sample project now includes a non-draggable marker (grey) and an alternative marker icon (red pin) to test all code paths
- GMDraggableMarkerManager adds a mapView property so that callers can clear a map from GMDraggableMarkerManager (e.g. to remove the gesture recognizer)

Cheers,

Stephen