Add HarbormasterRunnerWorker, for running CI tests

Summary:
This is very preliminary and doesn't actually do anything useful. In theory, it uses Drydock to check out a working copy and run tests. In practice, it's not actually capable of running any of our tests (because of complicated interdependency stuff), but does check out a working copy and try to run tests there.

Adds various sorts of utility methods to various things as well.

Test Plan: Ran reparse.php --harbormaster --trace <commit>, observed attempt to run tests via Drydock.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2015, T1049

Differential Revision: https://secure.phabricator.com/D4215