v1.5.21 - `plot_richness` / `estimate_richness` updates

phyloseq 1.5.21 - `plot_richness` / `estimate_richness` updates
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-- Additional alpha-diversity measures added to both functions.
-- New argument, `measures`, added to both functions, allows user to 
   specify which measures to calculate/display.
-- Also added unit tests for both functions, absent in prior versions
Addresses Issue 231:
https://github.com/joey711/phyloseq/issues/231