Put the thing that's actually changed in the changes hash
Merge pull request #2 from cddr/fix-changes-hash

Put the thing that's actually changed in the changes hash
Track dirty nested attributes
Only record changes on the association

The association is the only attribute we can be sure exists on the
model so I think it makes sense to give the association name to
the `attribute_will_change!` message. This gives clients of the
library a single way of checking whether some association has
changed regardless of whether it was changed by setting the assoc
itself, or the asssoc's ids or attributes (in case of
accepts_nested_attributes)
Merge pull request #5 from cddr/dirty-nested-attributes

Dirty nested attributes