Packaging updates (Sandcastle and NuGet)

Sandcastle documentation project updates

- Add a project reference to corelib to ensure that the SDK is built before the documentation project
- Enable the IntelliSense Component to generate a transformed XML documentation file
- Modify the syntax filters to use the same languages as MSDN: C#, VB, C++, and F#
- Modify the visible items filter to only include items visible from outside the library (no private or internal members, and document protected internal members as protected)

Updated NuGet package specification

- Updated description and release notes link
- Add XML schema so IntelliSense works with the nuspec file
- Explicitly reference the transformed XML documentation so IntelliSense information is complete

PS: To use NuGet specification IntelliSense, see the following Gist:
https://gist.github.com/sharwell/6131243