1.9.6 New `sortby` argument in `plot_richness` function

CHANGES IN VERSION 1.9.6
-------------------------

USER-VISIBLE CHANGES

	- New `sortby` argument in `plot_richness` function.

	- Sort discrete x by one or more alpha-diversity measures

	- Solves [Issue 342](https://github.com/joey711/phyloseq/issues/342)

	- Resolves/merges [Pull
343](https://github.com/joey711/phyloseq/pull/343)