Add labels to all exercises without them.

This way, we can hook them up with the HoTT repository.  Labels were
sometimes made by keyword-pulling, when I couldn't immediately think
of a good name for the exercise.  I don't care much what the labels
are, but I would like all the exercises to have labels.
Merge pull request #632 from JasonGross/label-exercises

Add labels to all exercises without them.