Fixes scroll issue in position:fixed elements

If the datepicker is placed within position:fixed elements, e.g. modals and fixed navbars,
scrolling the page also moves the datepicker away.

Tested when placed within:
- normal element, e.g. within a form
- fixed element
- modal

Tested on:
Chrome 25 (MacOS 10.8)
Safari 6 (MacOS 10.8)
Firefox 19 (MacOS 10.8)
Internet Explorer 7 (Windows XP)