Fix Raphael setAttribute errors

Summary:
Chrome 35 doesn't like calling

    node.setAttribute(att, " ")

which raphael seems to intend to be the same as calling

    node.setAttribute(att, "")

So I have replaced all setAttribute calls in raphael with calls to a custom `setAttr` function that strips the

Test Plan:
 * Manual inspection
 * Open perseus and make an interactive-graph and transformer, and check for errors in the console

Reviewers: eater

Differential Revision: http://phabricator.khanacademy.org/D9190