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 should be in its own `try` block, where a good recovery behavior is to print the usage.

The remaining content should be in a separate try since errors there are more likely due to failing jobs, rather than bad CLI invocation.  It may be better to just have `main` `throws Exception` and let this second half reside outside any `try`.