Better :focus support for .list-group-item

[sunabozu]

Hi.

It would be nice to have more transparent behaviour of list items when they're focusable. Basically, I want they automatically become .active when they're focused.

Plus I want to remove outline from focused items.

And, it would be really nice to display active but unfocused items with the grey background used in OS X (see below).

It's just a matter of cleaner code for developers.