preliminary version of clusters

Adresses issue #1573 . Preparation for clustering, with an example that clusters random points and prints the clusters to console. Things to be done:
1) Tests
2) Refactoring
3) Not clustering features outside the given extent
4) Recalculating clusters on zoom/extent change
5) Clicking on cluster should zoom to the extent of the clustered features
6) (maybe add support for more clustering algorithms?)

Any other suggestions are welcome.
```javascript
  for (var k=0, kk=cluster.length; k<kk; k++) {
```