new calc app

![image](https://cloud.githubusercontent.com/assets/1767466/4565889/df7a4bde-4f23-11e4-8195-080a218a4b47.png)

An initial crack at the calc app. A lot of the logic is in expression.js. Not sure how closely people do/do not want to look at this, as it's still in the early stages.

What this gets us:
(1) Translate blocks into an expression tree
(2) Ability to create a diff of the target expression, and what the student entered.
- It is smart enough to figure out that (1 + 2) and (2 + 1) are the same.
- It's not always 100% clear what that diff should be, and I may end up needing some tuning for cases where the expression trees look entirely different 
(3) Display the expression to the play area, marking parts that are wrong in red.

Stuff I'll need to do still
- There are lots of little todos in code I need to address
- Ultimately want to display expressions into play area one step at a time, possibly pausing when we hit a red letter.
- Figure out what feedback dialog looks like/what info we present there, given that the diff should provide us with a fair amount of feedback.