Fix sina plots

[clauswilke]

In the latest builds, sina plots have the wrong width. The simplest solution may be to write my own sina geom, based on geom_violin().

[clauswilke]

Actually, the simplest approach is to take stat_ydensity() (https://github.com/tidyverse/ggplot2/blob/master/R/stat-ydensity.r) and rewrite it so it jitters points rather than returning an outline. Technically this should be written as a position adjustment rather than a stat, but writing the stat will be easier. Drawing can be done with geom_point().