interactive-graph: ImprovablePoint!

Summary:
Creates Interactive2, the prologue of a remake of
interactive.js, and currently featuring MovablePoint2/
ImprovablePoint/Interactive2.MovablePoint

Test Plan:
 * Make an interactive-graph
 * set the type to "points"
 * try things:
    * try click-to-add points, make sure snapping works
    * try dragging a point on top of another point, make sure it doesn't
      let you
    * make sure the "correct answer" coordinates update correctly in the
      editor
    * smoke test grading
    * test the trash can in click-to-add-points
    * check the log for js errors

Reviewers: alex

Reviewed By: alex

CC: alpert, eater, justinjaffray, brianmerlob

Differential Revision: http://phabricator.khanacademy.org/D6865