add composer.json

I've added a composer.json file for alternative means of distribution.

Composer is a dependancy manager for PHP used with the Magento Composer installer it's a great way to manage dependancies.

This will work to compliment the modman file in #1. Modman is a popular script for Magento module deployment.

These two tools are ideal for users who manage their Magento project's through source control and not Magento Connect.