Updates to the C# Koans based on experiences with ~50 students

[elizabrock]

I've used the DotNetKoans to teach two of my classes the basics of C#. During the most recent class, I took careful note of the issues they ran into, so that I could contribute back fixes for future learners.

This pull request is fairly epic, and I understand if you don't want to merge in all of these changes. In particular, I'm not sure you'll want the changes to the .Net version. I was careful to keep the commits both topical and atomic, so I can easily remove any changes you don't want to merge in.

Thanks for your work on the DotNetKoans!