add delete-section button

For deleting all graphs on the same section easily.

![Screenshot_2013_03_26_21_11](https://f.cloud.github.com/assets/290039/302697/e9199f3e-960e-11e2-9171-59f59548c2bf.png)

![Screenshot_2013_03_26_21_16](https://f.cloud.github.com/assets/290039/302700/178f75a0-960f-11e2-883e-21de7da7978e.png)