Add stat logging middleware

Add middleware extra that will send request and async stats to a configured listener.

Example config

``` py
stat_processor: furious.extras.appengine.stat_processor.StatProcessor
stat_logger_connection:
  address: http://127.0.0.1:3000/api/tasks/
  protocol: http
  deadline: 2
```

@robertkluin-wf @johnlockwood-wf @tannermiller-wf @rosshendrickson-wf @macleodbroad-wf @tylertreat-wf