Add interface for $

I'm not sure if this is intended or not, there is no interface for [`$`](http://api.jquery.com/jquery/). I think every jQuery users will use `$` rather than long named `jQuery`. As I tested, it seems like `$` works fine.

If not to have `$` object is intended because of Scala.js's limitation (something like name-mangling?), I'll just close this PR.