[bug 727088] Exit survey processing 

A bunch of things here.

* Moved/adapted the code in `scripts/scripts/add_new_emails_to_exit_survey.py` into `kpi/cron.py`+`kpi/surveygizmo_utils.py`. This script has been running for a while on the sumo tools server. It makes sense to have it part of kitsune now like the rest of our cron jobs.
* Added the collecting of the exit survey results and storing them into the metrics table. This is called off the the same cron job mentioned above.
* Added API method to GET the survey results from the metrics table.
* Added a graph for that data. This was the easiest part (thanks @mythmon!).

I think that covers it from a high level. Ping me about anything confusing.

r?