Authenticate with Discourse via OAuth2

See https://github.com/michaelkirk/discourse_oauth2_example for an
example of how you might integrate your existing oauth2 provider's
authentication via a Discourse plugin.

db/migrate/20130816024250_create_oauth2_user_infos.rb

@@ -0,0 +1,14 @@
+class CreateOauth2UserInfos < ActiveRecord::Migration
+  def change
+    create_table :oauth2_user_infos do |t|
+      t.integer :user_id, null: false
+      t.string :uid, null: false
+      t.string :provider, null: false
+      t.string :email
+      t.string :name
+      t.timestamps
+    end
+
+    add_index :oauth2_user_infos, [:uid, :provider], unique: true
+  end
+end