Added description and unit tests to IO_ExtensionMethods_Folders.cs ( https://github.com/o2platform/FluentSharp/issues/10)

I tried to add also extra checks for some functions, hope it is ok . There is also a fix for a problem in String_ExtensionMethods.cs which I missed in the previous pull request.


Actually I just created a special issue just for this unit test (see https://github.com/o2platform/FluentSharp/issues/44) since this is a special type of test that requires more coding (and we still need to figure out how to replicate the White.Core.dll DesktopWindows method in FluentSharp's CoreLib)