Updating projects to use Docplagiarizer NuGet package

The "xml documentation comment copying build task" (trademark pending) had a name change, some polish and is now available as a NuGet package. So this takes care of replacing the old one as well as adding it to the Reactive project.

Yeah, DocPlagiarizer is what it's called now. Because naming projects should be left to someone else.

Some clumsy oaf managed to delete the remote branch of the previous pull request for this, so here is the remake!