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.


After I created some folder ( temporaryfolderpath) I was not able to delete it programmatically . It is always the case when you run visual studio without admin rights and in my case I was trying to save on root drive :).
 
Do you think it is ok to put this in create_Dir ?