1.9.1 psmelt bigfix

CHANGES IN VERSION 1.9.1
-------------------------

BUG FIXES

	- Bug in `psmelt` causing unnecessary error for phyloseq datasets with
empty components.

	- Solves Issue 319: https://github.com/joey711/phyloseq/issues/313
Merge pull request #326 from joey711/psmelt-rapidfix-1.9.1

1.9.1 psmelt bigfix

BUG FIXES

- Bug in `psmelt` causing unnecessary error for phyloseq datasets with
empty components.

- Solves Issue 319: https://github.com/joey711/phyloseq/issues/313