Add composer.json file for easy install via composer.
Composer: http://getcomposer.org/
Since HTML Purifier is not completely psr-0 compatible (a classmap is
not enough for autoloading), the package-description does not contain
anything autoload-related. The user has to include the autoloader
himself.
This lets us create an entry on packagist which allows installing HTML
Purifier without the need to declare a repository in projects; it also
makes it easy to create libraries which want to use HTML Purifier using
composer.
Signed-off-by: Benjamin Steininger <robo47@robo47.net>