Extend histogram bin auto-shifting algorithm to dates

[alexcjohnson]

We have an algorithm that shifts numeric histogram bins if a lot of the data is near the bin edges - for example, if all data are integers, we'll shift the bin edges to half integers. We should extend this to dates so that if for example all data is on the first of months, the bin edges get put at the middle of months.

This goes along with the desire to show more meaningful bin position hover labels - instead of a single x value as we do with bars, we want to show either both ends (perhaps clipped to what's actually in the bin?), or the single value represented if there is only one. Not sure if we have an issue for that already...

[alexcjohnson]

See TODOs added to histogram(2d)?_test.js in #1186