DO NOT MERGE Watch a demo form in modal

Implements @anothertompetty's demo form in a modal with new demo page.
This is a WIP – @harrison perhaps you could take a look at the approach.

![screen shot 2014-07-03 at 15 24 16](https://cloud.githubusercontent.com/assets/51724/3470807/c75003b2-02bd-11e4-98bf-45b22cee3122.png)
Can remove `data-target="#demo-video" data-modal-vimeo`