Nuspec template

Enhancement #28 
`ManifestFile` class has `Validate` method that otherwise causes:
`Error	1	The type 'System.ComponentModel.DataAnnotations.IValidatableObject' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.ComponentModel.DataAnnotations, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.	C:\Users\kovalikp\Workspace\Forks\nuproj\src\NuProj.Tests\Infrastructure\ManifestFileComparer.cs	8	18	NuProj.Tests`