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 with the previous case the ```checkIfDirectoryExistsAndCreateIfNot``` should be done  by the ```checkIfDirectoryExistsAndCreateIfNot```

In fact if you want, feel free to refactor that code from ```Files.checkIfDirectoryExistsAndCreateIfNot``` into extension method(s) since that **Files** class is from the time that I was not using Extension methods