Added composer.json

Added composer.json for installing and autoloading via [composer](http://getcomposer.org).
I am not sure, if you agree with the package-name and the description. If not, tell me.

For more information: http://getcomposer.org/doc/02-libraries.md

Example use:
--------------------
First another composer.json for the requirements:

    composer.json
    {
        "repositories": [
            {
                "type": "vcs",
                "url": "https://github.com/zyberspace/amazon-s3-php-class.git"
            }
        ],
        "require":{
            "tpyo/amazon-s3": "dev-master"
        }
    }

Then on the command line: `composer install`
Now we just need to include the vendor/autoload.php-file to use the S3-class in our project.

    test.php
    <?php
    require('vendor/autoload.php');
    var_dump(class_exists('S3'));