Updating the alerts in the rubric authoring.

Create an [X] button for canceling out alerts in the rubric authoring.

![image](https://cloud.githubusercontent.com/assets/235752/3707373/229c2c82-1434-11e4-8eff-f26b48dc58e5.png)


@wedaly @marcotuts @gradyward 
Thanks @frrrances !