Addition of Rackspace Queues

Hi there,

other than keeping an eye on Rackspace Load Balancers, it would be great to be able to keep an eye on Rackspace Queues. I've added this functionality into my "newrelic_rackspace_plugin"-fork (due to the fact it is no longer load_balancers-specific).

Thanks in advance for merging this PR, don't hesitate to get back to me for more information!

Kind regards,
David