ENH: Add numba engine for refine.
TST/API: Add tests and engine='auto' logic.

CLN: Removed numba-related code that wasn't needed in the end.
API: Expose engine and characterize through batch.
CLN: Fix file extension.
BENCH: Test locate with preprocessing off.
CLN: Allow float image input and make variables names better.
PERF: Tweaks improve speed when characterize=False.

Conflicts:

	trackpy/feature.py
WIP: Made a few refactorings to get a slight Numba speedup.
WIP: Removed np.float_ casts to get significant speedup
WIP: Additional un-numpying (marginal speedup)
WIP: very minor tweak to save one loop

Conflicts:

	trackpy/feature.py
WIP: Explicitly cast all input types.
Merge pull request #49 from danielballan/numba-refine

Add numba-accelerated refine/locate/batch through engine={auto, python, numba}