Ruler

Summary:
After playing around with this, I think a one-sided ruler (ticks only on the
bottom edge) combined with the pivot being a bottom corner would be better.

Test Plan:
Added `ruler({ticksPerUnit: 8, units: 10});` to a graphie div in
measuring_angles.html and tested the various options and functionality.

Reviewers: eater

Reviewed By: eater

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6535