Prepare for rejected 3.3.0

Consumer class change in methods to extend, implementing clients should implement process not _process.

Exceptions moved to rejected.exceptions

New consumer base classes:
- rejected.consumer.BaseConsumer
- rejected.consumer.PublishingConsumer
- rejected.consumer.RPCConsumer

New mixin consumer classes in rejected.mixins

rejected.data.Properties:
 - Change default content-type to text/plain
 - Remove access to cluster_id
Update examples