Support __getitem__ in TaxonomyDB
Emit taxonomy names rather than 'subject id' fields

- Require a path to the taxonomy database when running blast2json
- Use the taxonomy database to map GIs to protein taxonomy elements and
  emit names of the subject matcheskj
Rename TaxonomyDB.gid2tid() to gid2tid_prot()

- This method was only looking at the gi_prot_map file; adjust its name
  accordingly.