Queue classifications

Closes #48 
Can you elaborate on what issues you've run into? Ouroboros uses callbacks pretty extensively. Functionally there's no difference between code like this

```ruby
def create_something
   m = Model.create!(attribute: :value)
   m.do_something_else!
end
```

and 

```

class Model < ActiveRecord::Base
   after_create :do_something_else
end

def create_something
  Model.create!(attribute: :value)
end
```

I just think using an after_create callback makes the dependency of one action on the other more explicit and allows us to have less cluttered controllers.