Memoize class attribute reader on Authority::Abilities.authorizer_name.

Authority instructs you to include `Authority::Abilities` into all of your models.

As a rule, all of my models inherit from an `ApplicationModel` that subclasses the ORM I'm using. It handles all boilerplate inclusions, soft deletes, global callback hooks into observers, etc.

Naturally, I tried

    class ApplicationModel < ActiveRecord::Base
      self.abstract_class = true
      include Authority::Abilities
    end

to keep thing DRY. However, 

    module Authority::Abilities
      included do |base|
        self.authorizer_name = begin
                                 "#{base.name}Authorizer".constantize.name
                               rescue NameError => e
                                 "ApplicationAuthorizer"
                               end
      end
    end

calculates the class_attribute `authorizer_name` upon its inclusion in the model, meaning that without manual settings in every model (just what I was trying to avoid), `authorizer_name` gets set to `'ApplicationModelAuthorizer'` for all subclasses.

In the normal Ruby world, one would work around this as such:

    class ApplicationModel < ActiveRecord::Base
      self.abstract_class = true
      def self.inherited(subclass)
        subclass.send :include, Authority::Abilities
      end
    end

Rails is not the normal Ruby world, however. That approach results in the error

    NoMethodError: undefined method `synchronize' for nil:NilClass
      from gempath/activerecord-4.0.0.rc2/lib/active_record/attribute_methods.rb:26
      in `define_attribute_methods'

when instantiating any model inherited from `ApplicationModel`. This happens because somehow including instance methods triggers `define_attribute_methods` in Rails 4.0.0rc2 before the `attribute_methods_mutex` instance variable it references gets populated in AR core.

My fix is to memoize the getter instead of invoking the setter on inclusion. This pattern should be backwards compatible with all uses of `Authority`, in any framework, and is my weapon of choice for class_attribute defaults since it allows you to potentially set defaults based upon settings in `Authority.configuration` before `Authority.configure` has been invoked.