Added device_auth

Add to gemfile
gem 'spree_auth_devise', :github => "spree/spree_auth_devise", :branch => '2-1-stable'

1. bundle

2. bundle exec rake railties:install:migrations

3. bundle exec rake db:migrate

Edit spree.rb from: http://cl.ly/image/032A3M3I1v41
To: http://cl.ly/image/472q083E3d0Z

4. bundle exec rake spree_auth:admin:create