Add ClojureScript support

I haven't run any benchmarks yet so I don't know if it's significantly faster than the vanilla `clojure.zip` for ClojureScript. Either way, this makes life easier for things like CLJX. 

I had to change a couple of things to get the tests to pass in ClojureScript. `(identical? :end :end)` will return `false` in ClojureScript so `=` is used instead. And the `.property` getter syntax I changed to `.-`. That's pretty much all there was to it.