New props for measurer

Summary:
Changes the props `imageUrl`, `imageTop`, `imageLeft` to
`image: {url, top, left}`.

There are two motivations for this:
1. This makes the props somewhat nicer looking
2. More importantly, this gives us a widget with a new major version that
   doesn't exist in perseus-0, so we can test versioning more

This commit happens to make some of the code uglier (in my opinion), but
not horribly so, and I think the benefit of being able to test this on
a relatively benign widget will be very important.

Test Plan:
 * Made an item with a measurer and changed its image, top, and left
   parameters, and made sure that it displayed correctly.
 * Made the same item at http://khan.github.io/perseus/, and made sure it
   displayed the same way
 * Copied the item from http://khan.github.io/perseus/ (which is version
   0.0) to http://localhost:9000/test.html (which is at version 1.0),
   and made sure that it still rendered correctly, and that taking its
   JSON gave the new props with the correct values

Reviewers: joel, charlie

Reviewed By: joel, charlie

Subscribers: eater

Differential Revision: http://phabricator.khanacademy.org/D10896