Edit article user-interface update

Improved edit article dialog. Implements user-interface proposed in foodcoops#209 (leaving out subdivision).
![edit_article_improvement](https://f.cloud.github.com/assets/503804/1692522/6493e69c-5e7d-11e3-8464-e6b78c5894a3.png)

- [x] Update foodcoop price on keyup
- [x] Fix unit hint (override or update hint)
- [x] Fix validation with nested fields