Improvements

Hi,

I made a few improvements in the gem:

- Parent can access child attributes (parent.child_attribute)
- Child/Parent instances respond to instance_of, kind_of and is_a for the superclass as they were using inheritance
- Operator << is now usable in has_many relationships with acts_as_relation models
- Child instances can also access parent attributes using the regular object[:key] notation which makes the active record integration much easier)