Allow project to be used with Composer

> Composer is a tool for dependency management in PHP. It allows you to declare the dependent libraries your project needs and it will install them in your project for you.

I've added the `composer.json` file to this project to allow it to be used with [Composer](http://getcomposer.org/doc/00-intro.md). The [Composer documentation](http://getcomposer.org/doc/04-schema.md#authors) describes this file.