Transformer: better reflection tool

Summary: Adds a nicer reflection tool that can only be set to 45 degree angles.

Test Plan:
 * open test.html and add a transformer widget
 * Try dragging the line
 * Try rotating the line
 * Try dragging the reflection button
 * Reflect some polygons

Reviewers: alex

Reviewed By: alex

CC: eater, alpert

Differential Revision: http://phabricator.khanacademy.org/D6222