Add missing tests
Merge pull request #54 from djv/patch-1

Add missing tests
Add more tests
Fix Monad constrat on Bind instances for OptionalT.

Matches tonymorris/course answers.
Fixes #56.
Merge pull request #55 from djv/patch-2

Add more tests
Add tests for almost all StateT exercises
Merge pull request #59 from djv/patch-4

Add tests for almost all StateT exercises
Fix logging empty string instead of no logging
Remove answers
Merge pull request #60 from djv/patch-5

Fix logging empty string instead of no logging
Merge pull request #61 from djv/patch-6

Remove answers
merge