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.
the original repo just throws exception for unimplemented test. that really makes the output very messy to find the truly broken tests. that is why i add this attribute during the development.