Add ActiveRecord::Base#delete instance method

This patch adds an ActiveRecord::Base#delete instance method. It's like #destroy, but it doesn't call before_destroy callbacks and such.

This patch: - makes the API more consistent. I found it weird that there's Base.delete, Base.destroy, Base.delete_all and Base.destroy_all, but no Base#delete. - adds a plugin hook that I need. I also modified the autogenerated has_one/belongs_to before_destroy callbacks to call 'association.delete' instead of 'association.class.delete(association.id)'.