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.
I'm not sure what line you are linking to (88 and 188 both appear to be off-topic.)

Also, the syntax for links in markdown is `[Job.java](https://github.com/seomoz/qless-java/blob/master/qless-java/src/main/java/com/zimbra/qless/Job.java)`, where the url is placed in parenthesis instead of square brackets.

Line numbers can be selected in github urls by placing (for example) `#L88` on the end.

Finally, since files evolve, it is best to link to "snapshots" of files at specific SHAs by pressing the "y" key in the github file view.