Upgrade Sandcastle.

Sandcastle now has a Nuget package, so use that.  Update the dtfref project to work with the latest Sandcastle, and switch to the VS2013 presentation style.  Fix bug where the Sandcastle preprocessor variable was defined twice in WixBuild.wixproj.props.  Update Building Wix documentation.
Merge pull request #72 from rseanhall/sandcastle

Upgrade Sandcastle