Khanload refactor

Paired with distributed PR: https://github.com/learningequality/ka-lite/pull/1747
Moving central khanload code out of distributed and into central repository.
Still works!
![image](https://f.cloud.github.com/assets/4072455/2460116/2de3641c-af66-11e3-918f-66a5ccfbf012.png)