Transformer: Improved Line Grading

Summary: Grade lines as equal if they are collinear.

Test Plan:
Test that a line dilated or reflected such that the line remains the
same (but not the points) is graded as correct in test.html
Verify that other operations are not marked correct (such as other
dilations or reflections that do not preserve the line)

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5462