Replaced <small>'s css with adjust-font-size-to() based solution

I'm quite new to both compass and vertical rhythm, so I apologise if this isn't the right way to fix this issue, but but I noticed that large amounts of text within the small tag tended to break the vertical rhythm of a page.

After a little bit of research (by reading [this article](http://atendesigngroup.com/blog/vertical-rhythm-compass)) I came across the [adjust-font-size-to](http://compass-style.org/reference/compass/typography/vertical_rhythm/#mixin-adjust-font-size-to) mixin. It seemed that this, combined with a little bit of math, based around the `$base-font-size` variable, and I came up with the attached.

I hope you find it useful.