Add first version of pkg nestedset

* Allows creating nodes as sibling, child_of, and root
* Allows moving nodes to be sibling or child_of another node
Add \Nestedset\Model::as_parent_of()
Update README.md
Update \Nestedset\Model to have more static properties
Update \Nestedset\Model to have more static properties
Merge branch 'develop' of github.com:hubspace/fuel-nestedset into develop

Conflicts:
	classes/nestedset/model.php