Added Marker Clustering support

![screen shot 2014-01-28 at 11 17 13](https://f.cloud.github.com/assets/3678919/2017980/a3c2962c-8805-11e3-9995-1553b6552d5b.png)

Added optional functionality for grouping markers into clusters. Added a demo app too, with 2 maps showing a default and customized version.