Remove unused InverseDict import
Port PyCogent's InverseDictMulti

Only used in qiime/pick_rep_set.py, so easiest to port since we don't want this in skbio.
Move invert_dict based on @ElDeveloper's excellent comment
Remove unnecessary invert_dict import from tests
Merge pull request #1463 from ElBrogrammer/InverseDict

Port InverseDictMulti from PyCogent