Resolve #2, Clojurescript support.

[d-t-w]

• bump the Clojure version to greater than 1.7.0 (for cljc)
• add reader conditional to switch on clj / cljs
• bump .gitignore for intellij collaboration

[d-t-w]

relates to #2

[d-t-w]

Altered the key nature of percentiles map to aid destructuring.

[d-t-w]

Thanks for this useful little library @stuartsierra.

For my expedience I'm going to release my fork to Clojars under com.troy-west/frequencies, I hope you don't mind. I'll happily remove it if you're comfortable with the cljs port, i'm not too bothered about the int->keyword change it just suited my use.