Added composer for this project

With this, it can be installed as:
- create composer.json file with:

    "repositories": [

        {
            "type": "vcs",
            "url": "git://github.com/jacoor/woocommerce-rest-api-client-library.git"
        }

    ],
    "require": {
        "jacoor/WooCommerce-REST-API-Client-Library": "dev-master"
    },

and then:
curl -sS https://getcomposer.org/installer | php
php composer.phar install --dev --no-interaction
and you have api client installed in your project.