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

db/migrate/20110805201426_create_users.rb

@@ -0,0 +1,14 @@
+class CreateUsers < ActiveRecord::Migration
+  def self.up
+    create_table :users do |t|
+      t.string :email
+      t.string :password
+
+      t.timestamps
+    end
+  end
+
+  def self.down
+    drop_table :users
+  end
+end