Add an XML linter.

Summary: Add a linter which uses [[http://php.net/simplexml | SimpleXML]] to detect errors and potential problems in XML files.

Test Plan: Wrote and executed unit tests.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8989