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