Added ErrorsManager and Error(..., handler) extension methods.
Added license headers, missing documentation and reverted changes to ITracerExtensions.cs (which comes from the Tracer nuget)
Added Warn overloads. Improved NullErrorsManager.cs so that it returns NullErrorItems instead of nulls.
Improved usability by making the handler a simple Func<bool>
Merge pull request #18 from adalon/master

Added ErrorsManager and Error(..., handler) extension methods.