Mockup directories

So.....

`mockup.html` points to the reset CSS files in a stylesheets subdirectory, but all the files were actually in the same directory so weren't affecting anything. However, now the mockups look quite different (looks like `base.css` is the only one that really matters). Which one do we want?

## Original

![screenshot](https://f.cloud.github.com/assets/4689/1585606/9ce11860-521d-11e3-9b88-7b5f3701e7d3.png)


## New

![screenshot](https://f.cloud.github.com/assets/4689/1585596/8a623e08-521d-11e3-8198-e450cb7e794f.png)