Add worker to qless-java binding

The original implementation of qless-java binding does not provide "worker" concept like what have been done in its python/ruby binding. Inspired by [qless-py] (https://github.com/seomoz/qless-py), we implemented a simple serial worker to allow us for further Meerkat integration. Note this is a minimal implementation of [SerialWorker] (https://github.com/seomoz/qless-py/blob/master/qless/workers/serial.py) in qless-py. We still miss several advanced features like "signal", "resume" and etc.
This is a weird assertion, given that there's no way to actually use a return value in production.

There are other ways to prove the method ran, if you're just looking for an observable side effect.