Add gevent support for Producer

[mahendra]

This will be done as soon as #37 and #33 are merged. Should be few lines of code.

You can see the diffs here (on top of #37).
mahendra/kafka-python@gevent...gevent-producer

[dpkp]

There are no plans to use pluggable driver support. The current client code now has a very simple async io loop. I expect that if we were to implement a full async io stack, we would probably use asyncio.