Add map to problem_check event with descriptions of student answers

The map contains a human readable description of the answer if
necessary. It is useful for problem like multiple choice, when the
response of the student is replaced by a moniker. For example "choice_0"
instead of the full text.

Fixes: AN-587
Merge pull request #2721 from rocha/rocha/add-answer-values

Add map to problem_check event with descriptions of student answers