Added support for composer

Composer is a dependency manager for PHP that is highly used among a developers and their libraries. http://getcomposer.org/

This commit adds support for composer packaging for this library.

Please take notice that after accepting this pull request Disqus have to officially add the library in Packagist.
https://packagist.org/packages/submit

Signed-off-by: Alex Rico <alex@openhost.es>