CRM-13838: Allows report filtering by price level in ParticipantListing....

...php. Searches for price level options. Adds filter to _columns array. Left joins with line_item table and filters results by whether the contact record has a line item with a specified price level, but only if user has chosen to fliter by price level. Commented out 'fields' array because I wasn't sure if it was necessary for this filter. http://issues.civicrm.org/jira/browse/CRM-13838