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.


As before can you rename this to ```folder_Exists``` and create a 'marked as depreciated' folderExists() methods

We could also add a ```folder_Not_Exists``` method