Clustering solution for global maps

[peeb]

Currently the Google maps in Akvo RSR are quite dumb and simply drop a pin to respresent every location for every project and organisation.

In areas such as East Africa and India, the resulting maps are essentially unusable becase so many marker pins are stacked on top of one another at lower zoom levels that it's impossible to see what's going on until you zoom quite far in to get more detail.

A better solution to this is to cluster pins together at lower zoom levels.

[adriancollier]

Covered in #113