Make actual polling interval more accurate.
Make actual polling interval more accurate.

Previously, the time necessary to actually poll a check would delay the
next poll. Now the reference is the first poll, and the interval between
two polls on a given check should be closer to the specified interval.

Closes #8.

GuptaVineet-IBM/upTime_ford,b-pedzik/uptime,askcs/uptime,jraigneau/watchdog,memezilla/uptime,heamon7/uptime,nerevu/uptime,ifavo/uptime,Maluuba/uptime,SufianHassan/uptime,RafeHatfield/nodeuptime,erpframework/uptime,SpringerPE/uptime,rkmallik/uptime-openshift,0x8BADFOOD/uptime,blendlabs/uptime,SaravananPerumal23/uptime2,f33rx/docker-uptime,nacyot/uptime,blackboard/uptime,arthurpro/uptime,aregpetrosyan/web,traskat/uptime,brianjking/uptime,fzaninotto/uptime,bathree/uptime,scalp42/uptime,phanan/uptime,ptisp/uptime,gleicon/uptime,f33rx/docker-uptime,cloudfoundry-community/uptime-cfready,sinkingshriek/uptime,rkmallik/uptime-openshift,prune998/uptime,WillGuan105/uptime,ptisp/uptime,aregpetrosyan/web,nerevu/uptime,MichaelStephan/uptime,SaravananPerumal23/uptime2