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
Fix issue where deleting a user would show a 500 error in the JS console

Fixes #74
Account for scenario where Spree::BaseMailer doesn't exist in Spree <= 1.3.2
Bump devise-encryptable to version that limits devise versions to stable

Fixes #80
Properly close the method definition

Fixes #86
Merge remote-tracking branch 'github/1-3-stable' into 1-3-stable