Stick absolute elements

This pull request fixes a bug that happens, when an absolute positioned is sticked.

The following bugs are fixed:

* The spacer uses space it should not.
* When the bottom is reached, the top property is set to what it was before instead of ``auto``. At this point, it will have ``top: 0; bottom: 0``, which makes for a huge element. 

You can see the bugs in action here:

http://jsfiddle.net/caillou/FEKd4/

and the fixed version here:

http://jsfiddle.net/caillou/FEKd4/1/