Installation via Composer

Hey, [Composer](https://getcomposer.org/) has become a de-facto standard for installations of PHP libraries and tools. It helps you with managing and updating your project dependencies (and dependencies on another packages, though this tool has none).

I have prepared a `composer.json` file for you based on the information in this repository, but feel free to change anything before merging (as long the file stays valid).

There are three other recommended steps:
1. Register this package on https://packagist.org/ (automatic process based on your public repository and the composer.json file) - hassle free.
2. Enable Packagist Service hook on GitHub (provide API token from your packagist.org profile).
3. Release a stable version (1.0 ?), thanks to the hook this will automatically propagate to Packagist.

Thanks!