Custom progress element

This adds support for a custom `<progress>` element. There are a few hacks, all explained in the docs, that address Firefox and IE9 support. Should be ready to rock.

![screen shot 2014-06-10 at 2 47 40 pm](https://cloud.githubusercontent.com/assets/98681/3237467/e5a3b6b0-f0e8-11e3-94a6-e6436db39d48.png)