Removed Overlay interface.
Added OverlayMapType interface.

A GWT interface and javascript wrapper to be used for implementing map
types that can be overlaid on regular map layers via
google.maps.Map.overlayMapTypes.
Added an overload for Projection.fromLatLngToPoint

Projection.fromLatLngToPoint will create a new Point object if none is
provided.
Enhancements to map overlays.

Added a common Overlay superclass for Marker, Circle, Polygon, Polyline
and Rectangle. Added a few missing methods for draggability, visibility,
editability, etc.