Fix tooltip positioning with regards to viewports

The getPosition function was simplified with a new function,
getViewportBounds, was added to specifically
tackle the cases where we’re looking for the viewport bounds.

‘absolute’ and ‘fixed’ tooltips are now matched against the screen
where other ones are made to fit inside the viewport element.
@cvrebert  I wasn't too sure about that one, but based on http://jsperf.com/jquery-body-vs-document-body-selector this seemed like a good option to use `$('body')`.  Is there a good way to profile performance of jquery / bootstrap or in a case like this it's more a case of project standards?