stat_summary: implemented mean_sdl, implemented fun.y etc, allow own functions
stat_summary: implemented mean_sdl, implemented fun.y etc, allow own functions

Signed-off-by: Matthias Kümmerer <e388d34fd3c0456122779e95f262c0d70198a168@matthias-k.org>

kmather73/ggplot,benslice/ggplot,udacity/ggplot,bitemyapp/ggplot,benslice/ggplot,wllmtrng/ggplot,xguse/ggplot,Cophy08/ggplot,ricket1978/ggplot,ricket1978/ggplot,smblance/ggplot,xguse/ggplot,mizzao/ggplot,andnovar/ggplot,assad2012/ggplot,mizzao/ggplot