Introduce AMQ::Client::EventMachineClient::Deferrable#append_callback(&block)
Introduce AMQ::Client::EventMachineClient::Deferrable#append_callback(&block)

Semantics is the same as EventMachine::Deferrable#callback but instead of unshifting
argument to callbacks list it prepends it. Why? Sometimes we need FIFO execution
order.

travis-repos/amq-client,ruby-amqp/amq-client,travis-repos/amq-client,ruby-amqp/amq-client