Winphone81

- New solution, Raygun4Net.WindowsStore. This targets Windows Store Apps which run on Windows 8.1 and Windows Phone 8.1, and tablets built with VS2013

- Requires VS2013 and MSBuild 12 to build

- Asynchronous send methods along with async file & network access all the way down

- Handles sending manually instantiated errors, UnhandledExceptionEventArgs from the event handler marked as both async and synchronous, along with synchronous calls in catch { } blocks (compiler limitation, fixed in Roslyn)

- On UnhandledExceptionEventArgs, persists payload to disk for sending next time the application is loaded (Raygun4iOS behaviour) - change from WinPhone behaviour which stops main process from dying until it is sent (this doesn't seem to be possible while `await`ing async calls - the process straight up dies)

- Bump core SimpleJson file to latest 0.38