BeanIO for .NET as PCL

This is a hand-made port of BeanIO from Java to .NET which was changed in the following aspects:

* Support of .NET generics

* Availability as PCL (.NET 4.5, Win8, Win Phone 8.1, Xamarin.iOS/Android)

* Scheme handlers to support loading of mappings from any place

* Available as NuGet package

All original unit tests were ported (over 300) and should therefore mostly behave in the same way as the original BeanIO project.