Allow users to assign UUIDs to vertices

[mbroecheler]

FOR CONSIDERATION as triggered by #382.

Regarding UUIDs assignment:

1. If the user does not know what s/he is doing, allowing them to pass in UUIDs can cause a lot of havoc. I'd be willing to consider it as a feature for power users on very large graphs.
2. However, UUIDs would have to be longs otherwise Titan's storage format would become horribly inefficient. Also, this would mean that graph partitioning is up to the user since Titan uses the vertex id for partition assignment. Hence the question: Is it really worth it?

[mbroecheler]

I falsely closed issue #383 when I meant to close this issue with commit fc2bc83