Add a user_role factory

Naming the association in the model makes the relationship easier
to express in the factory.

app/models/user_role.rb

@@ -1,5 +1,6 @@
 class UserRole < ActiveRecord::Base
   belongs_to :user
+  belongs_to :granter, :class_name => "User"
 
   ALL_ROLES = %w(administrator moderator).freeze
 

test/factories/user_role.rb

@@ -0,0 +1,6 @@
+FactoryGirl.define do
+  factory :user_role do
+    user
+    association :granter, :factory => :user
+  end
+end