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.


where is that file created? I usually create them in the O2Temp dir that that is good

but yes, it is a good idea to move that code into create_Dir (since it will make that method more solid)