Installed and set up devise and omniauth.

app/models/user.rb

@@ -1,7 +1,19 @@
 class User < ActiveRecord::Base
+  # Include default devise modules. Others available are:
+  # :database_authenticatable, :lockable, :recoverable,
+  # :validatable, :timeoutable,
+  devise  :confirmable, :omniauthable, :registerable,
+           :token_authenticatable, :trackable
+
+  # Setup accessible (or protected) attributes for your model
+  attr_accessible :email, :password, :password_confirmation, :remember_me
   attr_accessible :display_name
 
   # user stamp
   model_stamper
   stampable
+
+  # validation
+  validates_presence_of :display_name
+  validates_uniqueness_of :display_name, :email, :case_sensitive => false
 end