support banker's rounding

[Josh Bleecher Snyder]

I'd like to add banker's rounding as a rounding option. apd supports it (RoundHalfEven), so it is a relatively trivial change, but it does mean adding new API.

WDYT?

[Bojan Zivanovic]

Sure, no reason not to.

Will you be using banker's rounding in your own project? I'd love to hear how/why. I didn't add the constant because I'd never seen it used in the wild, the eCommerce projects I consulted on always rounded either half_up or half_down.

[Josh Bleecher Snyder]

The why is kind of boring--longstanding habit plus avoidance of systematic bias.

I'll send a PR or two sometime over the coming weeks.