Affix: getScrollTop was returning boolean for bs-affix-target elements instead of scrollTop

Fixes issue #992

### Summary of issue
```getScrollTop``` returns a boolean when it should return scrollTop when ```bs-affix-target``` is used

### Code
The offending code

    function getScrollTop() {
        return targetEl[0] === $window ? $window.pageYOffset : targetEl[0] === $window;
    }

What it should be

    function getScrollTop() {
        return targetEl[0] === $window ? $window.pageYOffset : targetEl[0].scrollTop;
    }