Finish pull request for issue #50 (mapping markers to clusters, cluster items, and vice-versa)

Looks like @AntonioGonzalez disappeared.

My company would like to use this code in our current android project as soon as possible so I've taken the changes Antonio Gonzalez made and added the requested Map between Clusters and Markers.

Couldn't figure out a way to branch from Antonio's fork and then do a proper pull request on the main project, so I've included his changes as a separate commit and at least shared credit in the commit message.

The relevant code changes on top of what Antonio did are in 60b5501.