Add method to make arbitrary/custom REST API calls

[aseemk]

For flexibility, e.g. if node-neo4j hasn't implemented a REST API point you'd like to use (like the batch API), it'd be nice if there were a method to let you make an arbitrary call.

E.g. db.http('GET', '/db/data/foo/bar/baz', callback)

Or maybe db.get, db.post, etc.

[aseemk]

Getting tackled in the currently ongoing v2 redesign!

Issue: #143 / PR: #145
WIP docs: https://github.com/thingdom/node-neo4j/blob/v2/API_v2.md