hook in generator from controller generator

* use hook instead of invoke so it's easy to configure
* hook into controller_generator and scaffold_controller_generator
  This feels more natural than invoking from the resource generator.

Decorators are closely related to controllers, views and helpers.
So they should be generated in that context.
If controller generation is disabled for say generating resources there
is no point in creating decorators.
Readme updated to reflect new generator hook

You get a decorator whenever you create a controller. Still using 'generate resource' in the example to clearify this works from all generators that create controllers.
remove extra blank lines
Merge pull request #553 from azul/hook-generator-in-controller-generator

hook generator from controller generator