[#14] Teacher can change the limit of percent to succeed 

At the base, the default value was 70%
Now the user can change this limit to get a better idea of results.

#### *Example with 6 questions*

![70](https://f.cloud.github.com/assets/3521208/1844796/a2c467e2-7553-11e3-9ff8-7799b08ea39d.png)
![edit_limit](https://f.cloud.github.com/assets/3521208/1844797/aa9d9da8-7553-11e3-9778-22ea8cae4fd0.png)
![50](https://f.cloud.github.com/assets/3521208/1844798/ad78dd62-7553-11e3-9dfe-bfb974d45076.png)