Added composer.json

Added composer.json for installing and autoloading via [composer](http://getcomposer.org).
For more information: http://getcomposer.org/doc/02-libraries.md

Example use:
--------------------
First another composer.json for the requirements:

    composer.json
    {
        "require":{
            "jkonieczny/php-crontab": "dev-master"
        }
    }

Then on the command line: `composer install`
Now we just need to include the vendor/autoload.php-file to use the Crontab-class in our project.

    test.php
    <?php
    require('vendor/autoload.php');
    var_dump(class_exists('Crontab'));