(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 makes sense, but it also reveals some confusion about what we are talking about when we talk about "assessments" in our course design. Until now, I've heard "multi," "match," and this "open response" described to me to as "assessment types," but it sounds like these should more properly be referred to as specific "puzzle types" and not necessarily "assessments."

If we go by the implementation, we can call this refactored subclass DSL, since the other thing they all have in common is that their levels are all defined by a text-based DSL format.