Add authentication and User model

* Add User model
* Add SessionsController
* Add password authentication on User
* Request authentication for all actions except sign in
* Add some helpers for ApplicationController
* Update features to work with mandatory authentication

spec/factories.rb

@@ -8,4 +8,9 @@ FactoryGirl.define do
     mime_type 'audio/ogg'
     sha256 '94a5486a69a7261da350c57f9e5a1eaa789e08752cfc56a1989976a6ad82f7a8'
   end
+
+  factory :user do
+    email 'alice@example.net'
+    password '733tP4s5'
+  end
 end