generate reviewers stats for 1 and 3 years
generate reviewers stats for 1 and 3 years

Change-Id: Ib274166b49877838a1db20fcd84605c1bfc6b981

JohnGarbutt/reviewstats,openstack-infra/reviewstats,cp16net/reviewstats,JohnGarbutt/reviewstats,cp16net/reviewstats,openstack-infra/reviewstats