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.


this is an interesting test since this should be opening up a windows explorer dialog, which we will need to detect and close

We need to detect to confirm that it was actually created

We need to closed it, since it will be annoying for the user/dev if we dont