Gremlindocs methods tests, plus promises.

This PR adds the most of the tests in the [methods](http://gremlindocs.com/#methods) section of [gremlindocs.com](http://gremlindocs.com/).

But it also requires the experimental promises implementation of gremlin-node that has not yet
been incorporated into the [master gremlin-node repo](https://github.com/inolen/gremlin-node).

Currently this experimental promises implementation is only available here:
https://github.com/jimlloyd/gremlin-node/tree/feature/promises-api-2