Order orders by date.

Fixes #75
Fix session variable name for redirect_back_or_default

Fixes #79
Set user_mailer to use Spree::BaseMailer to mimic OrderMailer and ShipmentMailer

Set mail :from to from_address method from the base mailer, so that it will use the email in the current MailMethod

Fixes #77
Re-add destroy_user_session_path helper

Fixes #78