DelayQueue implementation, statuses and complex object serialization.

I've implemented DelayQueue based on your MySQLBasedDelayQueue. Implementation required code reorganization but I tried to keep backward compatibility.
What is more, I've added ability to set and fetch queue item statuses. They don't affect pull strategy and are maintained by user.
I also add ability to store complex objects using serialization.
More details could be found in readme.

I would be very grateful if you will find time to review my changes and eventually merge them.