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.


It is null because the folder does not exists.And it is almost impossible to fail since I added random letters to that folder name.
I removed the test because was pointless.