Get started: Server-side work and Y.GridModel

This PR adds some more functionality to the `/start/` route on the server-side:

* Looks at `req.query`, and generates sane defaults for any missing query params.
* Exposes a POJO to the client-side via `express-state`

On the client-side, I've created a `Y.GridModel` object that is responsible for keeping track of the Grid Attributes. It's also able to generate CSS based on the grid attributes.