Added MessageCount method to AdvancedBus

In my current project I've needed this functionality, so I've implemented it in AdvancedBus.
I know that EasyNetQ.Management.Client provides this functionality, but because of API low update frequency (minimum 5 seconds, as i know) it may not be useful in some cases.

Related SO topic: http://stackoverflow.com/questions/1038318/check-rabbitmq-queue-size-from-client