Center the MovableAngle starting vertex on (0, 0)

Summary:
Changes the default location of movable angles to be with a vertex at
(0,0), which makes creating and measuring obtuse angles much easier.

One not-nice thing about this is that it is a bit easier to position
the protractor "perfectly" on the angle without much thought (although
this is mostly due to the horizontal base ray).

Test Plan:
visit test.html
add an interactive graph
switch to "angle" mode
ensure it starts with the vertex on (0,0) [in the default -10,10 range]

Reviewers: alex, alpert

Reviewed By: alex

CC: eater, alpert

Differential Revision: http://phabricator.khanacademy.org/D5321