Allow error messages with non-ascii characters to be handled correctly

Also, add a test for this behavior.
Convert many byte strings to unicode; change string formatting
Fix some line lengths to make pylint happy
Fix formatting of docstrings; add more docstrings
PR fixes
Add tests for the diff coverage; fix one hidden unicode bug
Merge pull request #203 from edx/pbaratta/fix/capa-unicode

Fix unicode for errors on capa problems