Nuspec template

Enhancement #28 
Also, I think this should rather be an item instead of a property. That's because it would allows us in the Visual Studio extension to register a known item group so that the developer can select `NuSpecTemplate` from the drop down:

![image](https://cloud.githubusercontent.com/assets/5169960/4781285/8ab08206-5c95-11e4-93a0-a6dcd031af68.png)