Fixed left tooltip positioning

Hey @mdo, @fat,

I noticed a bug in left tooltips a few weeks ago:

![broked](http://i.imgur.com/5RXmTcn.png)

And poked around `tooltip.js`, resulting in the following:

![fixed](http://i.imgur.com/WeW21zC.png)

This patch gives left tooltips a similar treatment given to top tooltips: recalculating the left offset if the width changes after a class is applied.