Renaming *all* the exercises one last time

This removes the numeric prefix in the source files, while still
preserving it when populating the answers directory: in that case
the prefix helps direct the student from one lesson to the next.

But it should prevent us needing to rename everything every time
we'd like to change the order of something :)
Merge pull request #98 from stackgl/big-rename

Renaming *all* the exercises one last time