ggstance 0.3.1

This version updates all ggstance layers to feature parity with
ggplot2 3.0.0.

• New horizontal geom_colh() layer.

• stat_counth() is now exported (#22).

• position_stackv() gains hjust and reverse arguments (#17).

• New horizontal summary functions mean_se_h(), mean_cl_boot_h(),
  mean_cl_normal_h(), mean_sdl_h() and median_hilow_h() (#13,
  thanks to @rjbgoudie).

• Change minimal R version to 3.1.0, to be consistent with ggplot2.

In addition a few bugs have been fixed:

• geom_barh() now uses the counth statistic by default instead of
  count. This fixes many issues with this layer (#19, thanks to
  @erocoar).

• geom_violinh() can now be passed draw_quantiles argument (#16,
  thanks to @iamamutt).

• Fix horizontal boxplot issue when y scale is continuous.