Add a panel-footer class

Hi,

I've been using this in one of my work projects and thought that it might be useful to push upstream - it's a panel-footer class that acts much the same like the header, but just fits at the bottom. :)

The use case for such a class is somewhere to put buttons that is separate from the content. Here's an example signin box, using a panel, with a button that is pulled to the right:

![signinexample](https://f.cloud.github.com/assets/3307100/496455/e0e11d66-bbe2-11e2-962e-5040a1519c19.png)

The footer does not change depending on the panel style, as shown in the example below. I think it's better to keep it neutral.

![screenshot from 2013-05-13 23 37 26](https://f.cloud.github.com/assets/3307100/496468/4a528712-bbe3-11e2-83e3-c640750339c4.png)

I've tested it in latest Chrome and FF, and have no reason to believe it won't work in other supported browsers, since it's just a modification of footer-header.

Regards,
HawkOwl