Implements username authentication

It's now possible to login with either username or email
That's necessary to be backward compatible for iChain login

db/migrate/20141103132913_add_username_to_users.rb

@@ -0,0 +1,6 @@
+class AddUsernameToUsers < ActiveRecord::Migration
+  def change
+    add_column :users, :username, :string
+    add_index :users, :username, unique: true
+  end
+end