Grid system documentation

- [new] Rename column-height to row-height
- [new] Document the grid system variables and Sass functions

![screen shot 2013-12-12 at 15 02 50](https://f.cloud.github.com/assets/85783/1733895/7ec030fa-633e-11e3-8051-c6eac42f44fd.png)

![ ](http://s1.developerslife.ru/public/images/gifs/e020b43b-a8be-4156-b555-fbea5971d5ab.gif)