Include ActionMailer::DeliverLater::Mixin in ActionMailer::Base

closes #6 

cc/ @dhh @rafaelfranca 
Thanks . Btw how can i make this test pass
```
  test 'ActionMailer::Base should include mixin' do
    assert_includes ActionMailer::Base.ancestors, ActionMailer::DeliverLater::Mixin
  end
```
I added rails and require it but it still falling
but in an application i can see the the base was include the mixin