fix(position): Fallback for IE8's scrollTop/Left for offset

With IE8 and below, `$document[0].body.scrollTop()` and its left variant would always return 0, whatever the position of the scrollbar of the page is. This pull request add a third case when calculating the top/left offset with the `$position` service which service as a fallback for old IE. 

Normally, this allows directive based on `$position.offset()`to be correctly placed after scrolling. (tested with popovers and tooltip in IE7/IE8 compatibility mode)