Modularize the humanizer

It's now possible to extend the humanizer from outside of
foreman-tasks, by defining the subclass of Huamnizer::Resource.
Next step will be moving the resources related to Katello into
Katello (will do that after this one is merged) and remove the
Katello definitions once it's merged into Katello master.
Merge pull request #37 from iNecas/modularized-humanizer

Modularize the humanizer