Feature/structure node

Adds a IsStructureNode property to HasNodeInterface. This is useful if you want to have a special kind of node that is used to sort pages in place. You need to override IsStructureNode to true and IsOffline to true as well for it to work. A strcuturenode can't be given a slug or a url. It can't be published or drafted. It's just there for structure :)

As a side-effect of having to create a lot of nodes in migrations I ended up writing some support classes I call services. These belong to the NodeBundle and they are included here as well. One service to easily create pages and another one to easily add ACL permissions. They both work and are tested, but not yet in unittests.