Add a linter rule for extending Phobject

Summary: If I understand correctly, all classes should extend from Phobject?

Test Plan: This needs some further work

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13275