devise mailer 

closes #319 and pushes devise mail deliver to the background via queue. Let me know if i should break this into two PR's.

Notes: See commit log.
> Relies on sidekiq being available. Also this will change to when we bump rails 4.2+ and sidekiq to 3.2.4+. Currently blocked by Cellect client using prerelease of the Celluloid gem. See details on using deliver_later - https://github.com/mperham/sidekiq/blob/master/Changes.md#326