Composer.json added

Please merge this request to allow using your library with composer dependency manager (http://getcomposer.org/).
Additionally please register at https://packagist.org/ and submit your library via https://packagist.org/packages/submit. The last step should be creation of the Packagist service hook at https://github.com/zeroclipboard/zeroclipboard/settings/hooks#packagist_minibucket  It will allow to keep your package up-to-date in Packagist.

It would be great if you could add a tag to this repo after merging (it will be marked as a stable release on Packagist).