GitHub status API support, replaces commenting

We're using this on https://github.com/theforeman/foreman/pulls if you want to see a working example.

The evaluating markers were removed as the linking of commit statuses to individual SHAs means that we now naturally know which SHA we've evaluted.

Note that to write commit statuses requires push access to the repo, whereas any user could comment on a PR.  You can also delegate repo:status OAuth permission to the script instead and it runs with the privileges of a user with push access.  I've been thinking about leaving commenting partially enabled (just to report build failures and whitelist errors) which makes OAuth delegation less attractive, since it would need two different OAuth setups (one for statuses, one for commenting).