Performance test harness.  Closes #21.

* Measure 5k messages. Initial benchmark for under 5 seconds
* Set parallel tests to 1 so there are no Redis issues
Performance optimization - don't use struct encoding/decoding with Poison
Stats recording - create a background worker to save stats

* Convert Stats to GenServer
* Use cast instead of call for termination from worker to manager
Merge pull request #35 from akira/performance_harness

Performance test harness