Improvements from my fork

- Fix some minor pylint warnings (Consumer being already declared).
- Convert the examples from amqp:// over to pyamqp:// (long story short we had issues with the SSL library and other robustness issues with the librabbitmq-c library at $work).
- Fix Publisher.publish to not accumulate declare across calls.
- Fix Queue to not preallocate a default Exchange object if one was already provided.
- Improve examples.
- Add an example in the documentation for how to use a Publisher object (it's incomplete, but it's a start).