adds Identity model and migration

db/migrate/20150824144524_create_identities.rb

@@ -0,0 +1,11 @@
+class CreateIdentities < ActiveRecord::Migration
+  def change
+    create_table :identities do |t|
+      t.references :user, index: true, foreign_key: true
+      t.string :provider
+      t.string :uid
+
+      t.timestamps null: false
+    end
+  end
+end