Setup basic Revision History page

Sets up paper_trail tracking in important models

and add conference_id to versions for storing conference_id
of conference related objects  as metadata.This will help in querying
for versions related to conferences.

Fix issues with factories & event types initialization.Import paper_trail testing helpers
Fix bug: Creating an event creates a useless version with event update

Add revert and view changes features to revision history

db/migrate/20160524050538_add_conference_id_to_versions.rb

@@ -0,0 +1,5 @@
+class AddConferenceIdToVersions < ActiveRecord::Migration
+  def change
+    add_column :versions, :conference_id, :integer
+  end
+end

db/migrate/20160614145614_add_id_to_users_roles.rb

@@ -0,0 +1,5 @@
+class AddIdToUsersRoles < ActiveRecord::Migration
+  def change
+    add_column :users_roles, :id, :primary_key
+  end
+end