Adds an :on option to ActiveRecord observers

The would read much better if you could do:

observe :comment, :photo, :after => :create

-or-

observe :comment, :photo, :on => [:create, :update]

Thanks for the comments, Bryan!

The former, I agree with. When I get a moment I'll add that functionality to the patch.

The latter would be a problem because there's cases where you might want before_update rather than after_update. Defaulting it to use after_ by convention would cause problems in the case where a custom callback is being used - notify(:before_some_domain_specific_event) in your models, for example.

Here's a new patch that adds the :after => :create kind of syntax.

+1 this is nice syntax

having trouble applying the patch - will try to apply to master and modify as necessary