v2.4.0 - .NET Standard 1.2

• Switch to .NET Standard 1.2. Adds support for:
  • .NET Core 1.0+
  • .NET 4.5.1+
  • Mono 4.6+
  • Xamarin.iOS 10.0+
  • Xamarin.Android 7.0+
  • UWP 10.0+
  • Windows 8.1
  • Windows Phone 8.1
• Switch to Fubar.RestSharp.Portable
• More descriptive error handling
• Temporary: Remove Game.ProductCodeType because API returns incorrect response