Set up Composer

initial commit

Imported from MediaWiki core 1.24alpha

Source code:
* https://github.com/wikimedia/mediawiki-core/blob/4c01f8b/includes/libs/CSSJanus.php
* https://github.com/wikimedia/mediawiki-core/blob/4c01f8b/tests/phpunit/includes/libs/CSSJanusTest.php