Add timberline logger support. Defaults to a stdout logger.

With a Timberline logger we can allow any process running timberline to control the logger used by Timberline itself or any timberline worker. This will allow for a rails app to have timberline use the rails logger or any process running a worker to log through the default channel (which for our typical case, is STDOUT) or to determine where to send output. A good example of a use case for this is testing of workers in an app, which should output any logging to stdout when running as a separate process but should be not pollute the rspec stdout when running tests. 

Any thoughts or things I missed in how this could work? Solves a problem similar to https://github.com/treehouse/treehouse/blob/master/spec/lib/billing/recurring_billing_spec.rb#L4-L11. That particular case doesn't run through a worker but does represent the same problem.

/cc @wellbredgrapefruit