Allow progress bar to live inside of any element

This is useful in my case, where I have an admin panel-looking app, and I want to scope the loading animation to a specific pane.

I'm going to be honest, was lazy and did this PR on GitHub so I didn't run the tests.

However, I have a demo running here: http://jsbin.com/odOf/3

Fully willing to rebase this and resubmit, just let me know what you think. Thanks for this BTW.