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. You must also specify a `container-class` attribute with the class of the parent container that the affix'd element should not be able to scroll past (this is necessary otherwise we could potentially scroll off the page forever).

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.