Add callback for 'extentset' event.
Add callback for 'extentset' event.

florianf/tileoven,florianf/tileoven,paulovieira/tilemill-clima,paulovieira/tilemill-clima,tizzybec/tilemill,isaacs/tilemill,paulovieira/tilemill-clima,MappingKat/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,paulovieira/tilemill-clima,MappingKat/tilemill,Zhao-Qi/tilemill,MappingKat/tilemill,isaacs/tilemill,fxtentacle/tilemill,nyimbi/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,fxtentacle/tilemill,MappingKat/tilemill,tizzybec/tilemill,isaacs/tilemill,paulovieira/tilemill-clima,fxtentacle/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,mbrukman/tilemill,fxtentacle/tilemill,mbrukman/tilemill,nyimbi/tilemill,nyimbi/tilemill,nyimbi/tilemill,Zhao-Qi/tilemill,MappingKat/tilemill,tilemill-project/tilemill,fxtentacle/tilemill,Zhao-Qi/tilemill,mbrukman/tilemill,Zhao-Qi/tilemill,mbrukman/tilemill,tilemill-project/tilemill,tizzybec/tilemill,tilemill-project/tilemill,Zhao-Qi/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,tizzybec/tilemill,florianf/tileoven,nyimbi/tilemill,mbrukman/tilemill,tizzybec/tilemill