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.
check 33@[QlessTest.java] [https://github.com/seomoz/qless-java/blob/master/qless-java/src/test/java/com/zimbra/qless/QlessTest.java]. I can not see the value for taking a null jedis pool to initialize a Client but without the null check, that test will 100% fail. If that bothers you, I can ask the original author why he put this broken test.