FIX: Import bug was introduced in fit_powerlaw. Add test.

I'm not sure what change broke this -- I think it was a change in `api.py` -- but now there's a test to be sure we don't break it again.
I wrote the test before I made the change, so I know that this tests the change. The line `fit_powerlaw(em, plot=False)` is somewhat frivolous -- just one more smoke test to be sure the usage never gets accidentally broken.