Integrate ActiveJob / DeliverLater / GlobalID with Rails

WIP.
@jeremy so we make the `DeliverLater::MailMessageWrapper` a generic `MessageDelivery` and there we implement `deliver_later` which checks if ActiveJob is defined.
The initial implementation of the wrapper wasn't building the `Mail::Message` on creation (didn't had the `__getobj__` in initialize). There were some tests failing because they were testing behaviour from the mailer methods (ex: `assert_raises(RuntimeError) { LateInlineAttachmentMailer.welcome }`) and then we created the Mail::Message anyway. There were like less than 10 test cases so we can fix them (maybe using the new `#itself` method :) ). 
So to be clear: this implementation will change the behaviour of mailer. `SomeMailer.some_email` will return an `ActionMailer::MessageDelivery` which will be a `Delegator` around the `Mail::Message` which will be built lasy.