Add ResqueQueues plugin

This plugin reports the size of all resque queues. This plugin was originally submitted in pull request #87, but I have updated it to hopefully address the concerns that prevented it from being merged. I limit it to reporting the first 20 queues (sorted alphabetically). There is also a new setting, `queues`, to specify which queues to monitor. By default it reports the first 20 queues, but individual queues can be specified by name separated by a comma.

I have tested this plugin with an app that has 27 queues. I installed the plugin twice: once with `queues: '*'` to monitor the first 20 and the second containing the last 7 queues separated by a comma. Everything is reporting correctly and I can see the queue size for all 27 queues.