Breaking changes for sum / prod of empty / all-NA
Changes the defaults for `min_count` so that `sum([])` and `sum([np.nan])` are 0 by default, and NaN with `min_count>=1`.

I'd recommend looking at only the latest commit until https://github.com/pandas-dev/pandas/pull/18876 is merged. I'll probably force push changes here to keep all the relevant changes in the last commit until https://github.com/pandas-dev/pandas/pull/18876 is in, rebase on that, and then start pushing changes regularly.

cc @jreback @jorisvandenbossche @shoyer @wesm 