CRAN release

This fixes a few bugs, namely #31 and #30, and is on CRAN.

Bugfix #31: The default behavior of geom_beeswarm has changed. Before, if the x and y variables have the same number of points, the y-axis was dodged by default. The option to override this (groupOnX=) was lost in the method chain. Now, the option is restored and the default is instead to group on the x-axis. Also now if groupOnX is not specified, and this condition was triggered, a warning regarding the changed behavior is thrown.