Graph store

There is a small issue with the chain of iterables in the shared adjacencies method. We currently do not have a ClosableIterables.partition() method so I had to break the closeable iterabe at that point. It would be very useful to have a closeableiterables.partition method in Mango that could return a CloseableIterable<Iterable<T>>. Once this is completed, I'll change the shared adjacencies method accordingly.