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.


I don't think we need these privs here, but it will be good to add them to the create_Dir() extension method.

Btw, can you point me a scenario where you need to have this privs? since In windows when you create a folder you can always delete it