Add missing g.start method

If, for whatever reason, you use native graph manipulation routines,
converting the result back into something that gremlin-node knows how
to deal with can be done with g.start().
Update dependencies to their newest versions

Updates the following dependencies:
* Gremlin: 2.4.0
* Titan: 0.3.2

Adds the following new dependency:
* titan-lucene: 0.3.2