Build script now uses required MSBuild 12 (from VS 2013) to build wp81/win store pcl
Rename new PCL to WindowsStore
Guard against null sensor in Windows 8.1-targeting projects
No StackTrace class anymore so can't walk exception frames too see if it was from UnhandledException, so can only assume it wasn't

Users will need to pass in the args if it was, calling
client.Send(args.Exception) will no longer fire the ExitException if
they didn't handle it
Add to nupkg, and ensure NuGet references WindowsStore assembly when added to Win8.1 project

Instead of the the WinRT assembly