C#: Improve the out-of-the-box-experience

[Paul Rohorzka]

For people not used to using NUnit, the following things would be of great help:

• Provide a solution file
• Change csproj to a ClassLibrary project, remove then obsolete Main.cs
• Add nuget package for NUnit
• Provide a hint how to install the NUnit VsTestAdapter

I will send a pull request.

[Emily Bache]

Thankyou for the pull request that fixes these issues