added marker_size attr
added marker_size attr

ChakriCherukuri/bqplot,ChakriCherukuri/bqplot,dmadeka/bqplot,rmenegaux/bqplot,ChakriCherukuri/bqplot,bloomberg/bqplot,SylvainCorlay/bqplot,bloomberg/bqplot,SylvainCorlay/bqplot,rmenegaux/bqplot,ssunkara1/bqplot,dmadeka/bqplot,bloomberg/bqplot,SylvainCorlay/bqplot,ssunkara1/bqplot