Support auth via OAuth or personal access tokens

We used to use GitHub's tokens, but they seemed to have withdrawn them.

Checking their docs now, though, it seems you can create "personal access tokens" via https://github.com/settings/tokens/new which can be used in basic HTTP auth in place of a username and password: https://github.com/blog/1509-personal-api-tokens

So, if that still works, we should change the documentation to suggest creating a token (and listing the minimum access requirements - at a cursory glance it would appear the minimum we require is:

repo, public_repo, repo:status, read:repo_hook, write:repo_hook

Looks like this should be easier than I thought - it provides a way to avoid dealing with the horrible hell-spawned clusterfuck that is OAuth, which is kinda nasty for web apps and next to useless for non-webapps.