Fix incorrect offset from top when affixed element is affix-bottom.

Hi!

This fixes an issue caused by #12862.

Gif of the issue:
![before](https://cloud.githubusercontent.com/assets/4823640/2755710/a416ccda-c96c-11e3-9b68-f21a442c406f.gif)

Gif with my change:
![after](https://cloud.githubusercontent.com/assets/4823640/2755717/b3763fbc-c96c-11e3-89a5-f5053f9c8df9.gif)

Demo of twbs/bootstrap @ master:
http://embed.plnkr.co/lr1b2JjTtZYwpSFqJ7GL/preview

Demo of charlesbjohnson/bootstrap @ affix-bottom-use-bottom-offset:
http://embed.plnkr.co/FEePQwXnwMUCGm4suKOB/preview

Thanks!