Evaluated CSS asset URLs should be rewritten for all CSS clients

[joshuafcole]

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

The current rewriting system is only capable of rewriting URLs for the LT-UI client due to an inability to generically get the path to the context in which the CSS will be injected. I may have missed a field or function that is already available to that end, but if not this issue (and ensuing PR) should focus on providing such an abstraction and using it.

@ibdknox, you mentioned that you had some thoughts on how we should go about this?