Add async actionmailer

[passalini]

http://reefpoints.dockyard.com/ruby/2012/06/26/rails-4-sneak-peek-async-actionmailer.html

[RafaelMCarvalho]

They removed it from rails 4 😭

http://stackoverflow.com/questions/20270255/undefined-method-async-for-actionmailerbaseclass-in-rails-4

The solution would be use resque_mailer, but it's a big feature, so we are going to implement this just on next gem version. Or wait for rails 4.2, that will support async mailers built in.

[passalini]

@RafaelMCarvalho I think this is important like #30, because if the email wasn't sent, the method will not run too