Fix for .btn borders in IE7/8

Created by: necolas

Not sure if you've rolled this into your fixes branch yet, but I found a way to have the bottom-border-color fallback set in IE7 without getting a black border:

.btn {
    ...
    border: 1px solid;
    border-color: #ccc #ccc #bbb;
    border-color: rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.25);

And to save you some potential head-scratching, I just spent 30 minutes trying to work out why one set of button elements under "Example buttons" had a black border in IE7 once you set fallback borders...it's because they aren't wrapped in a form!