Implemented delayed messages

[pkruithof]

Changes

• Added support for delayed messages, via the delayed message exchange plugin
• Added ack/nack support to consumers
• Consumers now work via a callback that calls the given processor
• Consumers now dispatch events when consuming messages
• Added getter/setter for headers on Message class

Breaking changes

• Moved TreeHouse\Queue\Consumer to TreeHouse\Queue\Consumer\Consumer
• Consumers now work with EnvelopeInterface instances instead of Message
  instances, which are only for composing new messages.
• Removed MessageProvider and its interface, as it was basically a copy of
  the consumer class. Instead, consumers now work directly with a queue/processor.

[fieg]

lgtm