Add support for taxa

[gaurav]

We currently include taxa information in id-eq-id, e.g.

> GET "NCBIGene:100732537"
"[{\"i\": \"NCBIGene:100732537\", \"l\": \"Ssuh2\", \"d\": [\"ssu-2 homolog\"], \"t\": [\"NCBITaxon:10141\"]}]"

So we should be able to just modify the existing description lookup code to support taxa as well.

Issue #154 covers a comprehensive overhaul of this system, but it would be nice to add support for taxa since we have that data ready, and we only need to modify the description information to support this.