Custom Affix Directive

This is the only way I could think of to get this to work since Bootstrap’s Affix (and all other related plugins that use position:fixed) don’t work with fluid column layouts. As a result, I’m setting the top CSS property on every scroll event in order to mimic the appearance of position:fixed.

To use this directive, use add the `affix` affix attribute to any HTML element and specify an `offset-top` attribute that is used to determine when any element with the affix'd element should stop scrolling with the rest of the page and remain in a fixed position.

If you don't see any noticeable performance hits with this though (hopefully you shouldn't), then I think it would be okay to merge it into master.