Generate latency graphs for major routes based on BigQuery log analysis

Summary:
This should let us see how perf changes (both optimizations and regressions)
affect average users.

The graphs look like this:

{F55136}

Test Plan:
After running `pip install bigquery` and `bq init --project_id=khan`, run:

    gae_dashboard/generate_perf_chart_json.py

This will take a long time, since it's making a 14 days * 8 routes = 112 BQ
queries. Once it's done, re-run

    gae_dashboard/generate_perf_chart_json.py

And it should be really fast, because all of the BQ query results should be
cached.

Once that's run without error:

    cd gae_dashboard
    python -mSimpleHTTPServer 4321

Then open up http://localhost:4321 in your browser and you should see some
pretty graphs.

Reviewers: alan, chris

Reviewed By: chris

Subscribers: csilvers, chris, alpert, benkomalo

Differential Revision: http://phabricator.khanacademy.org/D12977