Transformer: Add a basic angle shape

Summary:
 * Adds an angle to the list of shapes that can be transformed
 * Does not support labeled angles currently

Test Plan:
 * Add a transformer and angle in test.html
 * Try all of the transformations on it
 * Try grading it for empty, correct, and incorrect

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5535