Move consumer classes from bin.huey_consumer to huey.consumer

This should make it easier to create Consumers in code without relying on the
huey_consumer script.
Fix consumer tests
Remove logging-related parameters from Consumer __init__
ConsumerThread: Sleep when huey is non-blocking, not the other way round
Merge pull request #56 from jbaiter/consumer_module

Refactor consumer-code into separate module