AMQP-CPP 2.1.0

_Changes in this release_

1. Fixed issue with the Deferred::onFailure() method, it was called even when no failure had occured
2. The Channel::setQos() method now supports a 'global' parameter, to indicate that the prefetch-count should be used as a total counter for all consumer on the same channel (when set to true), or that it is a counter that applies to each consumer individually.
3. Channel::get() method has been added to retrieve one message at a time.
4. Empty messages (messages without a body) were not correctly consumed.