Use requirejs for module loading

Summary: Also did some misc cleanup in khan-exercise.js, mostly related to module-y things ("151 ++----"). I combined local jquery-ui into one file to make it easier to load.

Test Plan:
Opened addition_1, deriviative_intuition, graphing_points_2 locally, and all looked fine.

In webapp/khan-exercises/, ran:

```
npm install -g requirejs
r.js -o requirejs.config.js
```

then saw that /exercises/addition_1 looked right.

Reviewers: eater, jack

Reviewed By: jack

CC: alex

Differential Revision: http://phabricator.khanacademy.org/D7095