Updated references to packages.
Corrected usage of Microsoft.Bcl.* packages (included the Build one).

Updated Digillect.Mvvm.Tests project to use shared assembly info.
Added missing copyright headers.
Improved NuGet package.
EntitySession removed in favor of Session with XParameters (entity key must be requested from Parameters object).
Parts mechanics changed to Actions/Execution groups.
Project housekeeping, fix for missing contracts and XML comments.
Release 2.1.0