Partial fix of Richness panel issues

Still need to add UI for max discrete classes,
and need to add the label-rotation UI
https://github.com/joey711/shiny-phyloseq/issues/32
Richness panel stability, efficiency

https://github.com/joey711/shiny-phyloseq/issues/32
Richness: add x-lab rotation ui

https://github.com/joey711/shiny-phyloseq/issues/32
Merge branch 'richness-issue-32'