Update migrate_from_trac.rake

Added check for time.nil? (otherwise error on rake task method not found)
Corrects time-reading, fixes error on issues-page
Tracker-Mapping, adds Task. Tracker now choosen by id, according to MY database (select * from tackers;)
Status-Mapping, added accepted. Status now choosen by id, according to MY database ( select * from issue_statuses;)
Added original db-numbers in comments, fixed resolved and added accepted status.
Changed priority-mapping for testing. Default = normal.
Changed role assignment.
Fixed wrong created_on date.