Report on feedback to exercises

[mquinson]

It would be very interesting if we could have a view about the feedback that the exercises got.
The students can give it when executing the exercise; here is an example of such commit:
BuggleInc/PLM-data@de64121

The possible fields are defined here: https://github.com/oster/PLM/blob/master/src/plm/core/model/tracking/GitSpy.java#L212
exoInterest and exoDifficulty are closed questions, with values defined here (it will always be in English in the commit):
https://github.com/oster/PLM/blob/master/src/plm/core/ui/ExercisePassedDialog.java#L81
https://github.com/oster/PLM/blob/master/src/plm/core/ui/ExercisePassedDialog.java#L88
exoComment is an open comment in addition to these closed questions.

I would love to discover what are the most ranked exercises, what is the average ranking that they get, what are the open comments for each of them and everything. I'm not sure of how I would like to see these information, but a big table of all exercises with a column of "#attempted", "#passed", "# interest ranked" "average interest", etc, is probably a good try.