faster heatmap row/col filtering
faster heatmap row/col filtering

fixes #649
filter all at once rather than cell-by-cell

aaronhoffman/dc.js,dc-js/dc.js,mtraynham/dc.js,aaronhoffman/dc.js,mtraynham/dc.js,TheDataShed/dc.js,mtraynham/dc.js,aaronhoffman/dc.js,aaronhoffman/dc.js,aaronhoffman/dc.js,mtraynham/dc.js,dc-js/dc.js,dc-js/dc.js,TheDataShed/dc.js,TheDataShed/dc.js,TheDataShed/dc.js,mtraynham/dc.js,dc-js/dc.js,dc-js/dc.js,TheDataShed/dc.js