Update to support pamqp 1.1 and add features!

- URL based connection strings
- Connection and Channel are now context managers (with connection as conn:)
- type keyword renamed to [noun]_type for all classes
- New feature for messages to automatically add timestamp and message-id properties if not passed in
- Message.ack and Message.nack new parameter "all_previous"
- New Queue.consume context-manager / generator
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