ClojureScript support

As part of my work to make [url](https://github.com/cemerick/url) ClojureScript compliant I have to add ClojureScript support to pathetic.
As pathetic is almost pure clojure (besides a Java Exception access) this is straightforward.

Still, there are 2 options:
* duplicating the whole clj file into a cljs file (no other change)
* use an external lein plugin (like [cljx](https://github.com/lynaghk/cljx): implies changing source code and project file)

What do you think?