Improved RabbitMQ message check.

- Check will now fail if it cannot connect to RMQ.
- Checks all queues and reports on warning/critical messages for each.
- Added `--ignore` option to allow us to specify queues to ignore.

(rebased as requested in https://github.com/sensu/sensu-community-plugins/pull/526)
It looks like this will never happen based on definition of CarrotTop.new https://github.com/portertech/carrot-top/blob/master/lib/carrot-top.rb#L10 So i would rather completely cut this rescue block with everything around and catch it in run section.