Add support for notifications when the connection is blocked

This both includes logging in EasyNetQ to make this situation more visible,
and exposes events in IAdvancedBus for clients to be able to respond to
the event.

See http://www.rabbitmq.com/connection-blocked.html for more information.
Merge pull request #276 from danielwhite/blocked-connection-notifications

Add support for notifications when the connection is blocked