Add sizzle for a lightweight selector, make the functions pluggable and shift the repo up a directory for child themes

Hey @peterwilsoncc,

This pull request might look scarier than it should be but I've done the following:

1. Added Sizzle for a lightweight but more robust selector engine.
2. Make the functions pluggable. (During which I realised I should have a child theme to demo the experience).
3. Move the repo up one directory so we can have a child theme to build up and demo stuff to each other as we go.

I have a feeling you'll want to question why I did this: https://github.com/peterwilsoncc/collaborate/commit/2a4093999cf5527817a2b02cd81978ed2c6a200a#L1R68 and https://github.com/peterwilsoncc/collaborate/commit/fb5d6fae020f79d6fdefc24aae85fe5859f840d5#L0R69 inside the after_setup_theme action but there is some method to my madness.

The method is (which I can expand on in our next hangout) that as we build up our base theme will be adding lots more filters and actions. If we keep adding them inside the collaborate_setup function we can simply copy and paste that function into a new child theme and delete all the filters/actions we don't want and we're rolling. It's a different take on what you used to do in Big Red with your $soup object. The good thing about this is that it lowers the barrier to entry for designers who are scared by php but get functions.php

You'll either love it and want to either :thumbsup: or :punch: me. I'm happy to :video_camera: and chat about this new approach.