Change Devise Mapping to be spree_user, rather than just 'user'

This stop spree_auth_devise from clashing with any other Devise-backed User-model using configuration from applications.

While this *may* break existing applications, the change is necessary in order to prevent mysterious behaviour.

The mysterious behaviour I have seen today was that an application was using
Devise for its own User model, and the application was also using
spree_auth_devise which provides the Spree::User model, but defines a Devise
mapping of :user.

Due to spree_auth_devise overriding the Devise mapping for :user, the wrong user
class was being referenced in an omniauth callback, leading to undefined method
errors.

Moving spree_auth_devise over to a 'namespaced' mapping is the only way to
ensure that strange behaviour like this doesn't happen and that
spree_auth_devise is completely separate from any other authentication setup
within the main application.
Only depend on core, frontend, backend + API components
Set default host option for AM in password_reset_spec
Add logout route. Without this the store_location method was throwing an exception.

Fixes #72
Add missing translation keys that were previously available in core
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
Use success messages in admin/users_controller
Fix issue where deleting a user would show a 500 error in the JS console

Fixes #74
Bump devise-encryptable to version that limits devise versions to stable

Fixes #80
Switch to spree namespace for translations
Fix request specs, failings down to 1

Add en.yml file with devise namespace entries. It feels odd to have
the devise namespace on spree core locale file since users can opt to
use other auth strategies or even have an already devise auth model.
Keeping those entries on spree core may confuse / override users
existing app devise locale entries

Fixes #85
Bumping Versionfile for 2-0-stable
master branch now depends on spree 2.1.0.beta [Fixes #95]
Versionfile clean up
Bump gem version
Speeling [Fixes #90]
use Spree.t on order.state [Fixes #96]
UserMailer inherit from Spree::BaseMailer

No need for check if the constant is defined since at this point users
tracking spree_auth_devise 1-3-stable must also be tracking spree
1-3-stable

Fixes #97