Added movable angle construction to interactive.js

Summary:
Drag ray controls points to move rays, optionally snapping to
whole degree angles. Drag vertex to move entire angle.

Test Plan: Verified expected behavior in Perseus widget.

Reviewers: jack, eater

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4392
Fix styles for MovableAngle.

math.js is currently loaded after interactive.js, so the standard colors
defined there aren't available when interactive.js is first parsed.

Test Plan:
Tested using the Angle subwidget in Perseus on local webapp and confirmed
that KhanUtil colors load correctly now.

Auditors: jack