Adds util for embedding images in emails

Summary:
This is the first piece of adding sparkline graphs to the email reports.
Unfortunately, Gmail won't display embedded SVG (and certainly not canvas/js),
so we will have to render the graphs to PNG, and embed those.

Test Plan:
Tested in a shell, putting images into an HTML body and sending it to myself.
The images display correctly in Gmail.

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: benkomalo, marcia, alan, chris

Differential Revision: http://phabricator.khanacademy.org/D11605
Save each day's report to disk.

Summary:
This is the next step in putting graphs in the emails.  We save each report to
~/bq_data/<report_name>_yyyymmdd.pickle.  (Tell me if there's a better place to
be putting the files.)

Test Plan: Tested in a shell, loaded the pickles manually and they look good.

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: alan, benkomalo, chris, marcia

Differential Revision: http://phabricator.khanacademy.org/D11606
Adds util to render a sparkline

Summary:
Right now, they're all black; eventually I'll add colors based on whether the
data Looks Bad.

This requires gnuplot be installed; how should I go about doing that?  (The
normal Ubuntu package should work.)

Test Plan: write the output to a file, open it as a PNG.

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: benkomalo, marcia, alan, chris

Differential Revision: http://phabricator.khanacademy.org/D11611
Modify sparklines to allow missing data.

Summary: This will be useful if a URL isn't hit every day, for examples.

Test Plan: Render sparkline in shell, write output to file, view it as PNG.

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: benkomalo, alan, marcia

Differential Revision: http://phabricator.khanacademy.org/D11620