Add note field to form type

I've added a note field to use the notation feature:

![capture decran 2014-03-05 a 14 00 54](https://f.cloud.github.com/assets/103900/2333190/87988344-a466-11e3-89fd-ca022bf867d3.png)

Note is also visible in admin comments list:

![capture decran 2014-03-05 a 14 04 37](https://f.cloud.github.com/assets/103900/2333199/bd8c39aa-a466-11e3-83ae-8cdd19f06149.png)