Allow tabs to fade in if no initially active pane is present

I can see the use case, do we want this?

Demonstration can be found [here](https://rawgit.com/hnrch02/bootstrap/dd501278e79e831e28fec91b1a77502489ff1994/js/tests/visual/tab.html).

/cc @mdo @fat
That's what I thought :wink: