Added Activity model, ConcertoConfig entry, and cron entry to achieve #871
Update 02-concerto_config.rb
Add conditional for 0 days for keeping activity logs
Found the proper classname to query-but obviously only under newer rubies
Whilst we have no tests for it, this would have been sure to fail under 1.8
Merge pull request #873 from concerto/871_purge_pa

Added Activity model, ConcertoConfig entry, and cron entry to achieve #871