Add Bantik's contributor code of conduct

@Bantik's contributor convenant: https://github.com/Bantik/contributor_covenant

Heard about it while reading the #atunit meeting minutes:
https://www.atunit.org/wiki/MeetingMinutes

It doesn't seem to be opinionated enough that it could be controversial. Adding it seems simple enough, and the real proof of it's effectiveness will be proved by our referencing back to it :)

(Worth noting that this applies to contributors, not users, so https://github.com/gittip/www.gittip.com/issues/1425 still stands.)