Add a React <Graphie> Component

Summary:
Adds a <Graphie> react component to provide a declarative interface for
adding graphie items to a perseus widget.

See src/widgets/example-graphie-widget.jsx for an example
See src/components/graphie-movables.jsx for the implementation of
MovablePoint
See src/components/graphie-classes.jsx for the implementation of
Graphie.createClass used to create MovablePoint
See src/components/graphie.jsx for the implementation of <Graphie>,
which renders the MovablePoint

Test Plan: Add an example-graphie-widget, and test grading at various points.

Reviewers: alpert, alex

Reviewed By: alex

Subscribers: brianmerlob, justinjaffray

Differential Revision: http://phabricator.khanacademy.org/D7994