Support adding custom headers to Neo4j requests

[aseemk]

E.g. an X-Request-Id to correlate db requests with app requests.

Along with setting defaults, e.g. customizing User-Agent.

Stretch goal: possible to add onto the driver's User-Agent?
Discussion: https://github.com/brian-gates/cypher-stream/issues/10#issuecomment-62127284

[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