Fatal crash because grade_anonymous cannot be found

If you just install the quick_edit block by itself it gives a fatal crash because the grade_anonymous class cannot be found. See attached screen shot.

I just added in a check to see if the class exists before actually calling the static method on the class.

![screen shot 2013-08-23 at 4 25 42 pm](https://f.cloud.github.com/assets/2071843/1019694/2e3b8dac-0c4d-11e3-924d-e0b55a54d0b2.png)