Make the plugin available on packagist.org

[heiglandreas]

Currently the plugin is not available via packagist.org which is a pity. I'd love to be able to use it in a composer.json-file.

[kadamwhite]

This plugin has been submitted to the WordPress plugin repository; we can consider other repositories if it does not end up being rolled into WordPress core, in which case composer registration would not be necessary.