Server rendering: rendering of entire document using React.

Summary: Allows rendering of React into the "document" as opposed to into a
particular node. To recap some basics:

document: One level above the <html> tag - like the browser.
document.documentElement: <body>

To support full-page server side rendering, we need to be able to render
*everything* including the HTML/BODY tags. This allows that.
adding better test - moving execution env module.
Merge pull request #289 from jordwalke/ServerRenderingFixes3

Server rendering: rendering of entire document using React.