Major rework for the C# API client

* Removed separate .NET 4.0 Project
* Added GenerateSecuredApiKey to Portable Class Library
* Now supports .NET 4.0, .NET 4.5, ASP.NET vNext 1.0, Mono 4.5, Windows 8, Windows 8.1, Windows Phone 8.1, Xamarin iOS, and Xamarin Android
* Updated code comments
* Switch from Travis to Appveyor for CI