librato-node should let you use it more transactionally

[vincentwoo]

Right now the only blessed use case is to start the worker and hope for the best. I would like to be able to (in a short-lived node script):

1. Not start the worker
2. Gather some data via librato.timing(...)
3. Call librato.flush with a callback that tells me when the data has been written to Librato, so that I can terminate the process.

Any chance you guys would be down for exposing librato.flush's callback?

[bobzoller]

sounds good. PR welcome!