Rename \Nestedset\Model to \Nestedset\Node
Fix some odds bugs in \Nestedset\Node

* Also add field 'level' for getting a path
* Add \Nestedset\Node::path() which returns path of the current node up to the ROOT-node (or the one highest in the tree)
Add \Nestedset\Node::path()
Add support for multi-root nodes and a tree-manager
Update \Nestedset\Node and \Nestedset\Manager for better interoperability
Update \Nestedset\{Node,Multinode,Manager}

* Add multi-root support throughout all classes. Needs to be tested
though
* Also add methods for path, tree, and root to \Nestedset\Manager
Update \Nestedset\Node::is_valid()
Update \Nestedset\{Manager,Node} with proper static/non-static declarations
Update \Nesstedset\{Manager,Node} back to proper static variable declarations
Add test-support for package

Also add model for testing the simplenested model as well as a first
test-case
Update Manager and Node for multi-roor configuration
Update Manager and Node to fix some bugs