Allow selection of extending documents

- Adds new multivalue node type property phpcr:classparents
- Popualtes this with the class names of ALL extending classes on
  persist
- QueryBuilder selects on phpcr:class OR phpcr:classparents
Refactored a little and added test.
Fixes some text
Refactored to get parent classes from ClassMetadata
Fixed terminology
Renamed method
Removed unused use
Merge pull request #218 from dantleech/select_extending_documents

Allow selection of extending documents