Added node order property

The node order property is automatically calculated on a new node. When
a node is moved the node order is updated on all of it and all of its
sibling nodes.

This has been implemented with an event subscriber.
Added builder for node order
Merge branch 'develop' of git://github.com/sulu-cmf/sulu into feature/node-order
Merge branch 'feature/node-order' into develop
Merge branch 'develop' of git://github.com/sulu-cmf/sulu into develop