ClojureScript compatibility

[greglook]

I've started to work on this a bit in the cljs-support branch. The main differences so far are:

• Declaring the type itself, as cljs has a different set of protocols and method names than clj does for the same roles, e.g. clojure.lang.ILookup/valAt vs cljs.core.ILookup/-lookup.
• Encoding and decoding base58 from bytes, which I've handled by creating a new cross-compiled library alphabase. Also includes many utilities for working with bytes generically across the two runtimes.
• Clojure can leverage java.security.MessageDigest to get native SHA1 and SHA2 implementations, while cljs doesn't have an equivalent. I'll probably omit them for now and rely on the backend for hashing, or users can find another library to support them.

[greglook]

Look into the Google closure library for support for hashing using goog.crypt.