Sort Richness Plots by Observed count

add an argument and the code to allow sorting by observed counts in the
richness plots
Update the Sortby Function

Use a generic sortby that can sort using sample_data or measurements.
Add a unit test to test each of those.
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)
1.9.6 New `sortby` argument in `plot_richness` function
1.9.6 New `sortby` argument in `plot_richness` function

Make sure plot is defined AFTER the factor level re-ordering for
`sortby`
1.9.6 New `sortby` argument in `plot_richness` function

Revise tests to reflect update.
1.9.6 New sortby argument in plot_richness function

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](#342)

- Resolves/merges [Pull 343](#343)