Add a linter rule to determine whether a class should be marked as abstract

Summary:
A class containing abstract methods must itself be marked as abstract.

PHP Fatal error:  Class X contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (X::Y) in /home/josh/workspace/github.com/phacility/arcanist/test.php on line 5

Test Plan: Added unit tests.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14559