Issue #14: Display chart of questions answered right/wrong at each level.

Added a graph image and table view of the distribution of right/wrong answers.
This information should help teachers interpret the numeric score's validity.

Fixes #14
Reformatted question-review headings for readability.

The tags and <hr> had been printed after the question, making them look like
a heading for the following question. Now the question number, its level, and
the tags are all printed as a heading for the question in the report.