Updated: use logging, fixed a missing import, rename variable
Updated: renamed variable in tests...
Re-order test files
Added: implement worker

There are still some small differences with the original queue_classic:
- connection argument of Worker initialization must be an instance of
  ConnAdapter. In the original queue_classic, a new object ConnAdapter
  is initialized using a standard connection. My ConnAdapter inherit
  directly from the standard connection object.
- the fork_and_work may not work: writing a test for it is a bit
  complicated since unittest catch also the SystemExit errors.

Notes:
ConnAdapter should be implemented like the original to avoid useless
reconnection.