replaced the incorrect definition of child_key properties

The old version created a property :parent_id without respecting the :child_key option.
Now it iterates over the field names given in the :child_key option ([:parent_id] by default) and creates the respective properties (with protected writer methods).
Merge branch 'master' of https://github.com/snusnu/dm-is-awesome_set
Merge branch 'master' of github.com:tillsc/dm-is-awesome_set