An initial pass at exposign the asynchronous nature of librdkafka up into Ruby

This should be enough to get some initial review of the API going. Hooking up a
Hermann::Result class to the Hermann::Producer in Ruby is in this commit but
actually driving the librdkafka reactor underneath is not yet hooked up.

Fixes #11
Fixes #15
Fixes #18
Merge pull request #20 from rtyler/issues/11-producer-feedback

An initial pass at exposing the async nature of librdkafka upwards