Add Mailer including Order Confirmation Email

The purpose of this PR is to provide a standard approach to emailing that can be leveraged throughout Sylius.

The implementation is modeled after the https://github.com/FriendsOfSymfony/FOSUserBundle. 
Order Confirmation Email is included as example use-case.

Looking for feedback if this is going in the right direction..

Also, I'm curious if we should find a way to (optionally) integrate the SettingsBundle, to be able to configure sender settings for each email via admin.

Thanks!