Add a linter rule to ensure that namespace is the first statement in a file
Summary: If a namespace is used within a PHP script, it must be the first statement.
Test Plan: Added unit tests.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D14526