(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)

ah, i see.. I think it makes little practical difference, but considering MultiDSL still inherits most of its implementation from MatchDSL, I'll leave this as-is for now- in a future refactoring pass I'd like to move the DSL parsing logic into these respective model classes, so it will be more consistent to maintain the same inheritance structure in both places.