Fix outputting of tab-delimited PICRUSt prediction

Previously, observation metadata field was hard-coded to "KEGG Pathways".
However, this changed to "KEGG_Pathways", but could also be COG based (COG_Category).
Even more complicated is that there may also be additional metadata fields (e.g "KEGG_Description").
Choice was made to output the metadata field containing .*Description (e.g. either KEGG_Description or COG_Description).
If KEGG_Pathway or COG_Category information is needed then user should output in BIOM format
and use categorize_by_function.py to get output in tab-delimited for the "level" they want.

Fix #114
Update predict_metagenomes.py testing files
Add better fallback for metadata in tab-delim output

In case the precalculated file does not contain a metadata field
with "*Description" in it, then just output the first obs_metadata,
and if no obs_metadata then don't output any.
Merge pull request #115 from mlangill/issue114

Fix output of tab-delimited format for predict_metagenomes.py