Labels for interactive shapes + static 2D drawing API for Graphie-to-PNG.

Summary: Added customizable, scale-invariant angle/side/vertex labels. Integrated support for these labels into the core interactive.js shapes. Created an API for content creators to draw 2D shapes based on the above.

Test Plan:
Play around with labels for subwidgets in Perseus's InteractiveGraph
and draw various static labeled shapes in Graphie-to-PNG.

Reviewers: alpert, eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D4253