(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)

This hasn't changed, Multi has always been a subclass of Match since the implementation of its model class is largely the same.