Added graphie.fixedElippse and other helpers for wrapping SVG elements in DOM nodes

Summary:
graphie.fixedEllipse can now be used to create SVG ellipses with a known fixed size.
These ellipses will have their own SVG canvas and return both the underlying Raphael object
and the DOM node (i.e., a <div>) in which it is wrapped.

See D14159 for more.

Test Plan: See D14159.

Reviewers: aria

Reviewed By: aria

Differential Revision: http://phabricator.khanacademy.org/D14158