Exception cleanup
Exception cleanup

- Rename`` RabbitPyWarning`` to ``AMQPException``
- Rename ``RabbitPyException`` to ``RabbitpyException``
- Fix the use of ``__repr__`` when it should have been ``__str__``
- Add a new ``RemoteCancellationException``

gmr/rabbitpy,jonahbull/rabbitpy,gmr/rabbitpy