Add reify_mongo to paper trail

PaperTrail Reify Mongo

# in config/initializers/paper_trail.rb
# you need to manually create versions
# you'll need to set version.object = YAML::dump(self.attributes) in your Mongoid Model
class PaperTrail::Version < ActiveRecord::Base
  def reify_mongo
    klass = Object::const_get(item_type)
    if defined? klass.mongo_session
      coder = PaperTrail::Serializers::YAML unless coder.respond_to?(:dump)
      attributes = coder.load(object)
      klass.new(attributes)
    end
  end
end