Include Birth_Date & Gender in orderby & filter of the participant report

In http://issues.civicrm.org/jira/browse/CRM-13279 David Greenberg added
birth_date and gender in the participant report. This commit includes
both of these fields in the orderby and filter. Also it adds two
configurable fields named 'age' and 'age at event' (calculated age at
the start of an event)