make geom_who names consistent

[schloerke]

geom_who should be more like gg_who. geom means you can "plus" into it... not supply a plot object

[hafen]

Yes thanks I was actually going to ask you to do this. We want it to behave like a geom I just didn't have time and this was before the extensibility stuff was added. We want it to be like a geom because it will usually be used in conjunction with other layers.