viewmodel updates for 4.0 preview 1

[christiannagel]

• src/Codebreaker.ViewModels/Models/Field.Serializable.cs#L49 possible null reference
• update nuget readme with new public types and methods
• add view-model tests to unit tests
• the new Serialize method returns a string, just use the new ToString method instead
• rename usings.cs to globalusings.cs
• add /// documentation to all public types and public members