Registering at Composer and packagist.org

In general, it is nice to have PEAR2 packages be available not just via Pyrus and PEAR, but via Composer as well.

This PR adds a composer.json, but for this package to *truly* be installable via Composer, it must be registered at packagist.org too, and ideally, with the auto-update hook in the repository as well.