Added association delete callbacks

You can now define a callbacks on a has_many or
has_and_belongs_to_many association that will be called
when an object is removed from the collection.
Merge pull request #103 from projecthydra/association_delete_callbacks

Added association delete callbacks
Add a test for association hooks on has_many relationships [log skip]
habtm#delete saves between the before and after hook.

When a has_and_belongs_to_many#delete method is called, the
relationships should be saved before the after_remove hook is called.
This enables the far side of the association to have the new
relationships in the after_remove hook.
Merge pull request #104 from projecthydra/save_between_hooks

habtm#delete saves between the before and after hook.