Docs design

Code review appreciated, but to see the visual style, you'll need to install and run the demos.  For the lazy, the top bit of the demo page looks like this:

![image](https://cloud.githubusercontent.com/assets/1735391/3725916/f9b83070-168d-11e4-87ff-d75298a5eb84.png)

I've kept the dependency on colors, but am now only using it for centrally standardised colours that have other use cases.  I propose if this is OK with everyone, we should remove the docs-specific use cases from o-colors.