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.


This does something different. This is used to append a folder to a particular folder. A use case would be 

```csharp
var a = "some folder"
var file_in_Subfolder_in_A = a.folder('b').create_Folder().path_Combine('file.txt').create_File('some contents')
```