Fix bugs in exam reporting

Some tricky stuff happening that made the display for the exam reports a little hard to understand. 

Now on the coachreports for tests we show the # correct/# attempted and the percentage correct (giving more visibility to the teacher or admin. 

![selection_034](https://cloud.githubusercontent.com/assets/1319950/3578693/d767ff6c-0bae-11e4-80c4-11148cc1acdb.png)

![selection_036](https://cloud.githubusercontent.com/assets/1319950/3578696/e42dd94c-0bae-11e4-9fec-78f8013e417c.png)