Init: check for jQuery instance as selector

This issue was originally brought up on Reddit: http://www.reddit.com/r/javascript/comments/2affb3/jquery_magic/

When `$` is passed in to `init`, it's read as a function, which is then passed through `init` again. I haven't touched the jQuery source before, so if this is a terrible place or way to solve the problem, let me know and I'll happily try again elsewhere/elsehow (?).