SharePoint 2010 - Programmatically create a folder in document librari

Maybe this was easy for everyone else but I have to admit that this wasn’t easy for me. As you may know I have developed a tool for SharePoint 2010 Site Provisioning in order to enhance the SharePoint development process which wasn’t able to programmatically create folders and subfolder in a document library or list. So while I tried to change this I had a lot of trouble to achieve my goal.