Extract useful utility functions out of gae_dashboard/email_bq_data.py

Summary:
This is the first step towards having nice perf graphs on a dashboard for
average route performance.

Test Plan:
Ran `bq init --project_id=khan`, authorized my account.

Modified `gae_dashboard/email_bq_data.py` to make _send_email a no-op.

Ran `gae_dashboard/email_bq_data.py`, saw it exited normally.

Opened up ipython, loaded one of the pickled files in `~/bq_data`, made sure it
looked reasonable.

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: alan, benkomalo

Differential Revision: http://phabricator.khanacademy.org/D12960