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.


The ```string.IsNullOrEmpty(folder) ``` shouldn't be needed here since that is already (or should be) done in by the ```isFolder()`` method