[2.0-wip] Adding contrast color for the focus

[benjaminach]

Hello,

Here is a fix to see the outline when a button is focus :

// Adding contrast color for the focus
.btn-primary:focus,
.btn-warning:focus,
.btn-danger:focus,
.btn-success:focus,
.btn-info:focus{
outline-color : #333;
}

[mdo]

Resolved this elsewhere for Firefox—noticed all colored buttons weren't getting the proper outline. Thanks!