Added a coffeescript implementation of watch.js

Hey, this is a coffeescript implementation of watch.js, which as best I can tell is what it was originally written in, since it compiled to almost exactly the same thing when I was done.

I wanted to submit this before I implement the `unwatchAll` function I mentioned in #164.

I'm not sure how you want to handle compiling this, it could either be done in the Gon source code, or you could precompile it by running `coffee -b -c -o ./js/ coffee/` which translates in english to `bare compile anything in coffee/ and save it into js/` (Where bare compile means to not wrap it in a `function() { ... }()` block as is coffeescript custom.

Let me know if you want me to change anything else.

Also, thanks for such an awesome gem, I use it in pretty much every rails project!