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.


same as above, this is already covered by the ```folder_Exists``` test