v1.5.16 improvements for `rarefy_even_depth()` function

Satisfies Issue 212
https://github.com/joey711/phyloseq/issues/212

phyloseq 1.5.16 - rarefy_even_depth improvements
===========
User-side convenience upgrades to rarefy_even_depth() behavior.

- More user feedback about process.
- New argument for setting RNG seed, with a default value (reproducible by default).
- Reports the RNG seed being used for random subsampling, encouraging users to record this for reproducibility.
- Trims empty samples/OTUs automatically, and reports this to standard out.