Clique module

Hi,

the patches add a clique module to ocamlgraph which contains the implementation of the Bron-Kerbosch algorithm for finding maximal cliques. It returns a list of cliques, which are just a list of vertices belonging to the same clique. There is also a second patch with a file for testing the implementation.

Let me know if there is something not in accordance with your implementation that I can change.

Thank you!

Best,
Giselle