Add a new theme 'ys'

A tested theme on my system and virtual machines.

For details please see [Introduction](http://ysmood.org/wp/2013/03/my-ys-terminal-theme/).

![ys zsh-theme mac](https://f.cloud.github.com/assets/1415488/252783/25a3055e-8bb4-11e2-9b9f-95c0d6f870fb.jpg)