Mixin class methods

test_mixin.rb

module Foo 
  def self.included base
    base.send :extend, ClassMethods
  end 

  module ClassMethods # the name doesn't matter as long as it match with line 3
    def bar 
      puts 'hello'
    end 
  end 
end

class Test
  include Foo 
end

Test.bar #=> hello