Add PersistAssociationTrait
Add association_class
Fix new_attribute_for for non Resource attributes.
Misc optimisation on persistor traits.
Fix parent key for compound class name.
add parents in persistable trait.
Clean persistor.
Implement deletable_parents for persistable trait.
Delete object before inserting similar one.

This is to solve unique key constraint problem.
old object needs to be purged before new one inserted.
Fix key not working for string and fixnum.