Nuspec template

Enhancement #28 
Good points. I think we should make `Title`, and `Authors` and `Owners` optional but validate that they are specified if no `NuSpecTemplate` was specified.

I would still require `Id` and `Version` as those are required for the package identity in the targets file itself. We *could* consider making them optional in favor of the values specified in the `NuSpecTemplate` but that seems more trouble than being worth.