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.


Since you are making changes to these methods could you:

1) add a new method called ```is_Not_Folder```
2) refactor the ```isFolder``` into ```is_Folder```
3) add a 'legacy' method ```isFolder``` that points to ```is_Folder``` (the idea will be to remove the isFolder sometime in the future