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
