Add support for Sucker Punch backend

Fala Grandão!

I don't know if you ever heard about Sucker Punch, but I think it deserves to be on devise-async. It is very useful for people who wants to have their emails being sent async on Heroku, but don't want to allocate a worker dyno and a redis server.

https://github.com/brandonhilkert/sucker_punch
http://brandonhilkert.com/blog/why-i-wrote-the-sucker-punch-gem/

Abração!