Activator: include instead of auto include.

No need to go around and stretch the code to not have methods available if we
haven't activated this for a model. As this will most likely only be used for
one or two models in a project, it makes more sense to have this being
included.

Instead of magically having `act_as_activator` - which is a horrible name
anyways - do all the work, we now use `include Activator` to get our methods
that we want.