CSS Asset Rewriting should be optimized to reduce complexity

[joshuafcole]

An Extension of https://github.com/LightTable/CSS/issues/1

The current system uses recursion, which is generally avoided in clojure due to a lack of Tail Call Optimization. This is a result of my relative unfamiliarity with clojure, so it'd be a good project to work on making it more performant and idiomatic.