add bg color variable for progress bar

This will allow developers more control to easily change the appearance of the default progress bars.

In my use case I removed padding and borders like so: `$progress-bar-border-size: 0;` and `$progress-bar-pad: 0;` and then wanted control over the bar color without adding extra classes. See screenshot for an idea of the result—which would be _super simple_ :grinning:  with this extra variable.

![Screen Shot 2013-03-06 at 1 17 18 PM](https://f.cloud.github.com/assets/196410/229302/e95ba98e-869a-11e2-93e8-0f1e75ad3439.png)