Adding composer.json to support compoesr-based installation

Composer (http://getcomposer.org/) is pretty much the standard and packaging and dependancy management for PHP projects and libraries. Providing a composer.json file in your library would be the first step towards allowing easy integration for modern PHP projects. 

You may want to edit some of the fields (e.g. version) and also add a "license" reference (couldn't find any license reference in your project). See https://getcomposer.org/doc/04-schema.md#license