Support reference biological sequence for each OTU

A new component slot, `refseq`, as been added to allow storage of any arbitrary type of biological (or otherwise) sequence, one for each OTU. Any object derived from the `XStringSet` class of the Biostrings package will work, most notably supporting DNA, RNA, and amino acid sequences, with the requisite import functions from fasta-format files also provided by the Biostrings package

This solves/completes [the feature request posted as issue 127](https://github.com/joey711/phyloseq/issues/127)