define Active Record Store accessors in a module

Allow store accessors to be overrided like other attribute methods,
e.g.:

    class User < ActiveRecord::Base
      store :settings, accessors: [ :color, :homepage ], coder: JSON

      def color
        super || 'red'
      end
    end
Merge pull request #9247 from lest/store-module

define Active Record Store accessors in a module