auto restore last attempt training

![screen shot 2014-09-07 at 22 55 35](https://cloud.githubusercontent.com/assets/4983239/4178625/2f710756-369f-11e4-837b-8bb14d17017b.png)

1. If a student haven't done a coding question, his code will be saved and auto restore next time.

2.provide a button if he wants to revert to the template code

3.UI is not beautiful.. need improve
if we need render multiple code in one page, that means there will be multiple 'prefilled', currently our prefilled is the only one. it's meaningless if I write a for loop to iterate all the CodeMirrors and put the same data.