Build integration: choose between global or per-project

[realvizu]

At the moment the MSBuild integration is done globally (in the Custom.After.Microsoft.CSharp.targets file).

This should be decided by the user (at setup time or later) whether he wants this kind of build integration or he would prefer per-project integration (adding NsDepCop task to the concrete csproj files). This latter should be assisted with some VS integration feature and/or NuGet package.

[realvizu]

The NuGet package enables per-project integration.
Now it's a documentation task to make it clear how to choose installer type and the difference between msi and NuGet install.

[realvizu]

Documented in Readme.md and Help.md