[WIP] index overflow bins in histograms

Addresses #227

@pwaller you are gonna like this!

This PR changes the histogram indexing so that index 0 refers to the underflow bin and index `nbins(axis) + 1` is the overflow bin. In the end this seemed like the best option. Allowing the user to decide the indexing scheme at init would just be a mess. Note though, that the methods that return a generator if index is None allow the option of excluding or including the overflow bins.

We will advertise this in BIG RED TEXT in the WHATSNEW.md and mail list. I'll start updating the WHATSNEW.md in this PR.