BCBREAK: Rename table nodes_taxonomies to model_taxonomies
BCBREAK: Extract Node::cacheTerm() as a Behavior
Refactor: Separate plugin model relationship setup

- Load Taxonomy behavior accordingly
- Remove hardcoded relationship, and instead set it up via behavior
Refactor: Do not depend on Nodes plugin
Refactor: Move Node::_parseTaxonomyData to TaxonomicBehavior

- New method TaxonomicBehavior::formatTaxonomyData()
- The above method is also called via Model.Node.beforeSaveNode event
Implement Vocabulary validation for multiple and required

Closes #298
Merge branch '2.0-taxonomy-refactor' into 2.0