Add a linter rule for abstract methods containing a body

Summary:
abstract methods cannot contain a body.

PHP Fatal error:  Abstract function X::Y() cannot contain body in /home/josh/workspace/github.com/phacility/arcanist/test.php on line 4

Test Plan: Added unit tests.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14560