Feature/pop with metadata

This PR resolves issue #33. @bakura10 please review and merge if OK.

Another thing is that I am unhappy with the way we test now the system which relies on Pheanstalk. It just clutters the test enormously. Perhaps there is a solution to improve, @ocramius can you give a suggestion perhaps? Please check BeanstalkdQueueTest#testPopPreservesMetadata().