Refactor: Extract method Node::saveNode()

- This change moves save logic from controller to model
- Extract several new utility methods:
  * formatData()
  * _parseTaxonomyData(),
  * getNodeRelativePath()
Add few tests for filterNodes().
Refactor: Extract method Node::_setCommonVariables()

Move common variable setup to a method
Refactor: Move update path logic to Node model

- Add 'plugin' key to _updateAllNodesPath()
- Add test for Node::updateAllNodesPaths
Refactor: Extract method Node::_findPromoted
Add quick tests for beforeFind / beforeSave callbacks.
Refactor: Extract method Node::_getType()
Refactor: Move processAction logic to Node model.
Merge branch '1.5-Nodes' into 1.5

Refactor Nodes plugin: moving logic to model.
Closes #321