highlight expression being evaluated

![image](https://cloud.githubusercontent.com/assets/1767466/4671140/c488e6a4-557d-11e4-8b2a-dde64a5ed903.png)

Show which expression is currently being evaluated by highlighting it in green. This may go away if we move to a future where Scrat is bouncing around to show where in the expression we are.

I also inverted the token list logic, such that tokens are either marked or not (instead of correct or not).