adding ability generator - closes #170

lib/generators/cancan/ability/USAGE

+Description:
+  The cancan:ability generator creates an Ability class in the models
+  directory. You can move this file anywhere you want as long as it
+  is in the load path.

lib/generators/cancan/ability/ability_generator.rb

+module Cancan
+  module Generators
+    class AbilityGenerator < Rails::Generators::Base
+      source_root File.expand_path('../templates', __FILE__)
+
+      def generate_ability
+        copy_file "ability.rb", "app/models/ability.rb"
+      end
+    end
+  end
+end

lib/generators/cancan/ability/templates/ability.rb

+class Ability
+  include CanCan::Ability
+
+  def initialize(user)
+    # Define abilities for the passed in user here. For example:
+    #
+    #   user ||= User.new # guest user (not logged in)
+    #   if user.admin?
+    #     can :manage, :all
+    #   else
+    #     can :read, :all
+    #   end
+    #
+    # The first argument to `can` is the action you are giving the user permission to do.
+    # If you pass :manage it will apply to every action. Other common actions here are
+    # :read, :create, :update and :destroy.
+    #
+    # The second argument is the resource the user can perform the action on. If you pass
+    # :all it will apply to every resource. Otherwise pass a Ruby class of the resource.
+    #
+    # The third argument is an optional hash of conditions to further filter the objects.
+    # For example, here the user can only update published articles.
+    #
+    #   can :update, Article, :published => true
+    #
+    # See the wiki for details: https://github.com/ryanb/cancan/wiki/Defining-Abilities
+  end
+end