Use a mutex-free logger.

Originally discussed in #877, because the licenses are not compatible,
I've instead released a mono_logger gem that is under the original ASL.
Now, Resque can depend on it, and we don't need to worry about the
licensing issue.

That said, I'm also working with @danielsdeleo to ask OpsCode about
releasing mono_logger under the MIT license, but this is a better
archetectural decision regardless.