simplify others grouper
simplify others grouper

by passing in rest of items, something we did not know before
this is a consequence of #1269

should be backward-compatible for anyone using a custom others grouper,
since it's just extra information being passed in

this makes the others grouper self-documenting, so we don't need the
bogus and vague custom others grouper example

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