Use a mutex-free logger compatible with Ruby 2.0

Imported Dan DeLeo's `MonoLogger` for Ruby 2 support.

This class was taken from [Chef](https://github.com/opscode/chef). The pull request introducing it says:

<blockquote>Ruby's stdlib Logger wraps all IO in mutexes. Ruby 2.0 doesn't allow you
to request a lock in a trap handler because that could deadlock. This
commit fixes by replacing the Logger with a lock-free variant.</blockquote> -- danielsdeleo

This fixes the warnings when running Resque's test suite in Ruby 2. They happen when Resque tries to log from within interrupt handlers.