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

closes #6 

cc/ @dhh @rafaelfranca 
Lets keep both requires. Who is using this gem will want to use activejob and actionmailer anyway. I just point about the on_load hook because on rails we will have to do differently.

About the tests I don't think it is possible to make this test pass without loading a real application. I'd remove it, but when integrating this code on Rails we could add a check on the railties tests.