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.


temp_Folder as  bit more complex behaviour that would be good to test here

for example when you call ```temporaryFolderPath.temp_Folder()``` and temporaryFolderPath is a real path, what happens?

what happens when it is a simple string?

what happens when it is partial path?

what happens if it is a real file path?