Add JSO wrapper for MarkerClustererPlus

The is a JSO wrapper around [MarkerClustererPlus](http://google-maps-utility-library-v3.googlecode.com/svn/tags/markerclustererplus/)

- It (mostly) only implements the parts of the API that I needed. e.g. I haven't done anything with events.
- It's numbered to match the version of MCP that it builds against, rather than the version of gwt-maps
- It's currently dependant on gwt-maps 3.9-b17 because that's what my project is using.

I'm happy to rework things if needed, just let me know.