1.7.22 psmelt bug fix. Protect against various name collisions.

CHANGES IN VERSION 1.7.22
-------------------------
USER-VISIBLE CHANGES
	- Revised psmelt to automatically modify data column-name conflicts,
with warning
	- Udpated `psmelt` doc to formally notify users of these potential
conflicts.
	- This solves Issue 307: https://github.com/joey711/phyloseq/issues/307
Merge pull request #310 from joey711/psmelt-col-issue-307

1.7.22 psmelt bug fix. Protect against various name collisions.