Color chooser

I thought that we might need a color chooser for categories (and labels maybe).

This one is dead simple and offers more or less the same functionality (and color palette) as GitHub:

![debug-3879](https://cloud.githubusercontent.com/assets/33978/4051355/dca44d90-2d60-11e4-855c-e3050e3d9255.png)

Ref #448