Upgrade to librato-java 0.1.0

librato-java 0.1.0 has some code cleanup and also abstracts away the HTTP behavior behing an HttpPoster interface. This update to metrics-librato adapts to this version of librato-java along with allowing for an HttpPoster to be supplied to the LibratoReporter.Builder.

- Ability to supply own implementation of HttpPoster
- Uses Ning's Async HTTP client by default

cc @mihasya @mheffner @josephruscio