Add possibility to remove triples.

Hi Ruben, still on the removal of triples described in issue #23. We've implemented something ourselves as we needed it for our project. Just sharing in case you find it useful. We tried to match your code style as much as possible. The implementation is very close to the addTriple method. To remove a triple, we remove the leaf in the 3 indexes as well as branches when there are no more leaves. We decided not to touch the entities map as we never remove entities themselves but rather modify the links between them. I think this is also fail safe...

Btw, congrats on the 3 cyclic 3-layered indexes, pretty darn efficient :).