Adds support for IronMQ.

* Added `connection_type` configuration method, allowing users to specify a `Backburner::Connection` subclass;
* added `auth` configuration method, providing generic auth options for a `Connection` subclass instance;
* modified `Connection` instantiation across the project to include `:auth` dictionary arg;
* added `IronMQConnection` class;
* added test to ensure `IronMQConnection` was authenticating properly.

Notes:

No testing is done to ensure that auth is successful; this is tricky with a `Pool`.
IronMQ does not support the following Beanstalkd commands: release, bury, touch, peek, peek-ready, peek-delayed, peek-buried, kick, list-tubes, list-tube-used, stats, stats-job, stats-tube, pause-tube, quit.

See http://dev.iron.io/mq/reference/beanstalk/ for more information.