Port InverseDictMulti from PyCogent

Changes:
- removed usage of `InverseDict` (was an unused import)
- ported `InverseDictMulti` (renamed `invert_dict`) as this was only used in `qiime.pick_rep_set`. Didn't modify the function from what it was in PyCogent, and decided it wasn't useful enough to add to scikit-bio. Can add there if you guys think it belongs there