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.
https://github.com/ericmartel/bootstrap/commit/7228c8071369b405d9426f66cb47733fcd8981b4#diff-f3e99e0bb007ace7a370f0492b9cb5abR355 That's how it was before, @ericmartel removed the `$.extend` per my suggestion, but I actually was talking about the one farther down this file.