Migrate build system from NAnt to MSBuild

The former NAnt scripts caused the assemblies to be bound to the current .NET installation (4.0 or 4.5).
Now they are compiled targetinjg always .NET 4.0.
Version number for the NuGet package has been increased on patch level.
Merge pull request #6 from CHECK24/MigrateToMSBuild

Migrate build system from NAnt to MSBuild