Add registered_by_id to Participant API

While creating reports using the API I realized how useful it would be to include the "registered_by_id" field as part of the civicrm_participant_search() results. My report has a column for "registered by" and having the id will make it easier to query further details. I'm still in the process of converting piles of direct sql queries to API and it's getting much better thanks to you guys!

I made this modification to civicrm/CRM/Event/BAO/Query.php to include this field.. see attached patch