0.2.0

• Update Henson-SQS to use asyncio coroutines for sending and receiving
  messages as required by Henson>=0.5.0 (Backwards Incompatible)
• Register a message acknowledgement callback to delete the incoming message
  from the queue after processing has finished
• Remove the SQS_DELETE_MESSAGES_ON_READ setting
• Make queue URL and AWS credentials settings optional
• Bugfix: postpone internal message queue creation until it's needed by the
  _consume function
• Bugfix: _begin_consuming needs to return the future, otherwise the exceptions are not bubbled up to Henson