Implement Node.traverse() to allow custom graph traversals.

[dkushner]

This PR implements the Node.traverse() method, satisfying #102. Total test coverage is forthcoming and it currently does not check for the 'fullpath' return type since that doesn't seem to be supported anywhere else in the API.

[aseemk]

Thanks for submitting this, @dkushner! Just to clarify, test coverage was added, right? Is there any more you plan to add to this PR?

[aseemk]

Btw, you're correct that fullpath return types aren't supported anywhere else in the API:

https://github.com/thingdom/node-neo4j/blob/1.1.0/lib/util.coffee#L151-L152

This is because Cypher's PATH doesn't return fullpath, so aside from this traverse method, there's nothing else that needs it.

[aseemk]

Sorry for the delay, @dkushner. The tests fail locally for me, do they fail for you too?

If you merge the latest master into your branch (or if you know git well, rebase your branch to the latest master) and re-push, Travis CI is properly configured now, so we can see whether they pass there.