Add Milestone information in Merge Requests

Step 1:
* Add milestone_id in the model for merge_requests
* Make it possible to create or update a Merge Request with an attached
  Milestone detail
* Add the possibility to filter by Milestone and / or Assignee in the
  Merge Requests listing page

db/migrate/20121026114600_add_milestone_id_to_merge_requests.rb

@@ -0,0 +1,5 @@
+class AddMilestoneIdToMergeRequests < ActiveRecord::Migration
+  def change
+    add_column :merge_requests, :milestone_id, :integer, :null => true
+  end
+end