AntonioGonzalez' changes to fix #49
Added map btw Cluster and Marker to DefaultClusterRenderer

This is to make the getMarker method for Cluster<T> faster than
iterating over all the markers in the mMarkerToCluster cache.

Added code to update the mClusterToMarker map each time a marker is
added or removed from the mMarkerToCluster map.

Fixed up some whitespace (to use spaces vs tabs).