Allow construction of incomplete LatLngBounds objects.
Allow binding of MVCObjects to other arbitrary MVCObject types.
 - Added some methods missing from the wrappers for various Overlay types
  - Rectangle: setBounds, getVisible, setVisible.
  - Circle: getVisible, setVisible.
 - Created an Overlay interface, implemented by Circle, Marker, Polygon, Polyline, Rectangle.
   Facilitates bulk manipulation of overlay visibility and parent map.
 - Created wrapper for Icon "class" and added getters/setters to Marker and MarkerOptions.