Support connection.blocked (coming in RabbitMQ 3.2)

This is not an issue but rather a heads up.

There is a [new feature in RabbitMQ nightlies](http://rabbitmq.1065348.n5.nabble.com/New-AMQP-0-9-1-extension-added-in-default-master-td28640.html) that requires client support. Java, .NET, Erlang and Ruby (Bunny, amqp gem) already support it their development branches, with librabbitmq-c and 2 Python clients having some progress on it.

It would be nice if the Go client had support for it before 3.2 comes out (which won't happen for a few more months). I'd be happy to help with it but unfortunately not in the near future.