TST: Test for MNLogit with strings/factors.
ENH: Make sure MNLogit works with string/factor y
TST: Silence tests.
REF: Refactor for DRY. Handle numpy-like like numpy.
STY: Remove commented code. Pep-8
Merge pull request #2048 from jseabold/mnlogit-factor

ENH: MNLogit strings/factor