Style the maintenance mode page

Fixes #64 by styling the maintenance mode page. 

I've included the global header and footer, but the message is left deliberately different to a normal page (based on the styleguide's look).

![image](https://cloud.githubusercontent.com/assets/1784740/4108835/4a109b24-31dd-11e4-9797-f31084f737b4.png)

It would be useful to refactor this in the future to avoid duplicating the header/footer HTML.