Throw error when flot.time plugin is missing.
Throw error when flot.time plugin is missing.

Fixes issues 709 on google code.

wet-boew/flot,chaveiro/flot,saike/flot,MAMISHO/flot,cipix2000/flot,kongx73/flot,gdseller/flot,soenter/flot,mlitwiniuk/flot,k-joseph/flot,andig/flot,Ninir/flot,chiu/flot,k-joseph/flot,andig/flot,stanislavdavid/flot,magora-suchkov/flot,resnick1223/flot,andig/flot,saike/flot,cipix2000/flot,colinhunt/flot,TrainingPeaks/flot,Ninir/flot,fashionsun/flot,k-joseph/flot,lolifans/flot,resnick1223/flot,ni-kismet/engineering-flot,wet-boew/flot,sedran/flot,pepperdata/flot,soenter/flot,stanislavdavid/flot,chaveiro/flot,cipix2000/engineering-flot,flot/flot,matong/flot,klarkc/flot,chiu/flot,octavianCraciun/engineering-flot,chiu/flot,Sensirion/flot,stanislavdavid/flot,mlitwiniuk/flot,saike/flot,petrvacha/flot,trask/flot,mcanthony/flot,chaveiro/flot,mlitwiniuk/flot,resnick1223/flot