Version 0.4.0 -- Promises & various enhancements

This PR is intended to be committed upstream to the reference repository [inolen/gremlin-node](https://github.com/inolen/gremlin-node), and then published to npm as v0.4.0.

While this PR itself only includes minor changes, it is based on several PRs staged in [jimlloyd/gremlin-node](https://github.com/jimlloyd/gremlin-node), which enhance gremlin-node in several ways, with the most significant addition being support for [Promises/A+](https://promisesaplus.com/) via the npm package [Q](http://documentup.com/kriskowal/q/).