V1 of the image widget, which allows labelling of user-provided images.

Summary:
V1 of the image widget, which places a (blank) Graphie component
behind the user-provided image. Allows custom scaling and addition/deletion of
labels, which have their own custom coordinates and alignment.

Test Plan:
- Run Perseus.
- Test on some [graphie-to-png](http://graphie-to-png.kasandbox.org/) images.
- See [here](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20image-widget%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22image-widget%201%22%3A%7B%22type%22%3A%22image-widget%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22backgroundImage%22%3A%7B%22url%22%3A%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F869c0d9c9103440c3d541cc458f3fbd4d4e380a8.png%22%2C%22width%22%3A321%2C%22height%22%3A320%7D%2C%22box%22%3A%5B321%2C320%5D%2C%22labels%22%3A%5B%7B%22content%22%3A%22Hello%22%2C%22coordinates%22%3A%5B0%2C2%5D%2C%22alignment%22%3A%22right%22%7D%2C%7B%22content%22%3A%22Goodbye%22%2C%22coordinates%22%3A%5B-5%2C3%5D%2C%22alignment%22%3A%22left%22%7D%5D%2C%22range%22%3A%5B%5B-10%2C10%5D%2C%5B0%2C5%5D%5D%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%5D%7D) for an involved example.

Reviewers: jack

Reviewed By: jack

Subscribers: alex, eater

Differential Revision: http://phabricator.khanacademy.org/D9298