(Msm assessments) First pass at new "Open Response" assessment type

Added a new "Open Response" level type:
> ![image](https://cloud.githubusercontent.com/assets/56541/4778039/9d962570-5bdb-11e4-8cc3-283f69c95787.png)

The level builder uses a simple text-based format to enter a name, title and prompt text:
> ![image](https://cloud.githubusercontent.com/assets/56541/4778031/742cfa24-5bdb-11e4-8f89-2aaa0c05e461.png)

The puzzle view is very basic: Display the prompt, and collect an input into a text area, that is saved as the "program" parameter and always marked as a passing submission:
> ![image](https://cloud.githubusercontent.com/assets/56541/4778034/88faff28-5bdb-11e4-9141-2888166b3fae.png)

I wonder if Assessment is the best name for this, since our model currently allows levels of any type to be marked as "assessments" in the script.

If in fact we want Assessment-ness to be strongly tied to being of Match/Multi/OpenResponse type maybe we should change the way we mark assessments in scripts.