[changelog] updated changelog for 0.9.8
[changelog] updated changelog for 0.9.8

bf4/sensu,rbramwell/sensu,withnale/sensu,miyakawataku/sensu,somic/sensu,npe9/sensu,keymone/sensu,roobert/sensu,javrasya/sensu,solarkennedy/sensu,sensu/sensu,crewton/sensu,cread/sensu,yonglehou/sensu,m4ce/sensu,quodlibetor/sensu,pauloconnor/sensu,zuazo-forks/sensu,Tapjoy/sensu-mirror,portertech/sensu,upfluence/sensu