Publish SDK as NuGet package

Publish the SDK dlls as a NuGet package so that we can easily reference from solutions in a standard way. 

Would also be great to publish Visual Studio templates to create new extensions, with the template including Package Manager and a minimal Spotfire runtime so that a developer can work without any external dependencies. We've created something similar so that we can (a) add developers without any manual steps (just get the latest repo code and open in VS) and (b) build and publish our SPKs using Visual Studio Online build processes without the need for any unusual build agents - the repo pulls down everything that's needed.