More accurately identify logged in users

Summary:
Changing the incoming data format means that I needed to change all of
the parsing code and deal with merging some data fields, so I did some
refactoring to make that easier. Now, it should be easy to add series to
this graph for other "avg" metrics from Google Analytics as well, like
avgServerResponseTime / speedMetricsSample.

I also fixed a small bug where we were assuming the wrong sample counts.
In practice, though, speedMetricsSample and domLatencyMetricsSample seem
to match most of the time.

Test Plan:
I configured ga_client_secrets.json and ga_client_privatekye.p12 as
instructed in the Khan/aws-config repo's analytics/setup.sh, then ran
./main.py -d and loaded the page as localhost:5000/is-ka-fast-yet. I
verified a few data points by hand against the raw data returned for the
same query by http://ga-dev-tools.appspot.com/explorer/

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D6605