VSSDK NuGet packages

Use the VSSDK packages from NuGet to manage references to assemblies in the VS SDK.

Minor project file organization to match some other projects and explain the custom elements.
Yes, it uses code from an [unversioned assembly](http://tunnelvisionlabs.github.io/vsbase/docs-master/html/edbfd3ce-43f4-4f3f-a90c-bc22bda19fae.htm) in Visual Studio 2013, but isn't actually providing any functionality at this point.