feat(affix): provide affix-target option

It some cases affix elements needed within scrolling div instead of window.
I solve this problem, here is example with modified affix service: http://plnkr.co/edit/WPfHrO?p=preview
I propose add new directive, which sets the scrolling target where need to listen scroll event and to get dimensions of the scrolling area.

Moreover, it helps to write unit-tests, now you can create scrolling element and perform checks there because you  may scroll it to any side as you want