Rename project to indicate it only supports Visual Studio 2012
Add new project targeting Visual Studio 2013
Use project-specific NuGet configuration files
Add proper support for separate extensions for Visual Studio 2012 and Visual Studio 2013
Configure separate extension names for the Visual Studio 2012 and Visual Studio 2013 extensions
Add License.txt and include in VSIX
Only deploy the extension during a build on the expected version of Visual Studio