Fix rabbit-queues check

This check had 3 issues. The first is that it was checking again :min
and not :expected, hence would always fail.  Similarly, there is no
fall-through case when > 0 and not within the critical threshold.

Also adding a -i argument so that we can quiet down some noisy queues -
in particular, q-plugin.
Differentiate between critical and warning

Right now the rabbit queues check is broken in that it only fires
criticals and is checking the wrong parameter for the high water mark.
Update this check to provide both warnings and criticals.
Merge pull request #415 from craigtracey/rabbitqueues

Fix rabbit-queues check