Add taxonomy as a valid string

When using taxonomy as a header key in parse_classic_otu_table, this
function would fail. Fixes #866.
Modified qiime/pick_otus.py so reference based OTU picking (usearch 5.X and 6.1) will create the failures file, even if there are no failures to write to address issue #868
Merge pull request #870 from walterst/gregs-edits

Modified qiime/pick_otus.py so reference based OTU picking (usearch 5.X ...
Merge pull request #867 from ElDeveloper/issue_866

Add taxonomy as a valid string