Progress bar

Adds an optional, customizable, in-browser progress bar to display page loading progress.

Seen here in Chrome:
![progress-bar-chrome](https://cloud.githubusercontent.com/assets/5355/4721418/b8d690e0-5938-11e4-890a-abaecff9397b.gif)
Also tested in: [Safari](https://cloud.githubusercontent.com/assets/5355/4721477/18530c24-5939-11e4-8ffc-48d20e55e247.gif), [Firefox](https://cloud.githubusercontent.com/assets/5355/4721516/47389dc4-5939-11e4-8f19-d5f2d6acb38e.gif), [IE 11](https://cloud.githubusercontent.com/assets/5355/4721522/53e25bbe-5939-11e4-99b1-45b5309e49dd.gif), and [IE 10](https://cloud.githubusercontent.com/assets/5355/4721532/5eee657a-5939-11e4-8194-b6fa5deaccd3.gif).