Refs. #29. The test are actually for the v0 version of the redis engine.
Separate the engines unit tests.
Refs. #29. Add tests for the new Redis engine.
Move the functional engines tests to a separate package.
Refs. #29. Change the way we do one-step reports in redis engine:

 - Use `hincrby` even if the key doesn't exists yet. Redis
   automatically creates it and sets it to 1.
 - Use `hsetnx` to set the entered date, this only sets it
   the key doesn't exists.
 - And use `hset` to always set the updated date.
 - Change the decode method to expect some keys to be missing
   from the hash.