Added a ruler to Interactive Graph + Protractor -> Measurer

Summary:
I still want to make some changes to the actual ruler component,
but the Perseus integration won't change much.

Test Plan:
Tested Interactive Graph widget with ruler, protractor, both, etc.
Tested Measurer widget with different combinations of options.

Reviewers: eater

Reviewed By: eater

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6536