Transformer: Add a fancy rotation handle

Summary:
Adds a fancy curved-arrow rotation handle for the rotation tool, similar
to that on the protractor.

Test Plan:
visit test.html, play around with the rotation tool
(enabling, disabling, moving it, rotating it, switching tools)

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5269