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).
Merge pull request #67 from jakeonrails/master

Finish pull request for issue #50 (mapping markers to clusters, cluster items, and vice-versa)
Make methods to get ClusterItem and Markers public

As discussed here https://github.com/googlemaps/android-maps-utils/pull/67
Merge pull request #95 from lukesleeman/patch-1

Make methods to get ClusterItem and Markers public
removed import com.google.android.gms.maps.model.MarkerOption as its no longer needed
Merge pull request #108 from perlethandersen/patch-1

removed import com.google.android.gms.maps.model.MarkerOption as its no ...
ConcurrentHashMap backed sets to avoid concurrent modification exceptions.
ConcurrentHashMap style changes
Merge pull request #109 from bobzilladev/concurrent-hammer

ConcurrentHashMap backed sets to avoid concurrent modification exceptions