Added Retina support
Added Retina support

Closes https://github.com/PistonDevelopers/piston/issues/516
Closes https://github.com/PistonDevelopers/piston/issues/517

placrosse/piston,bvssvni/piston,oldmanmike/piston,mitchmindtree/piston,caldwell/piston,PistonDevelopers/piston,tempbottle/piston,getynge/piston,KevinFrans3/piston,bvssvni/piston,PistonDevelopers/piston,Potpourri/piston,axelmagn/piston