Add straw

The farming mod should be completed by a straw node that allow another use of wheat, and makes nicer the farms with a traditional look.

![straw](https://cloud.githubusercontent.com/assets/9265590/5539117/d8ca8bc0-8abd-11e4-98e7-5ab82b407e19.jpg)
You don't need this line because it will automatically drop itself unless stated otherwise.