Add composer.json for project

Hello,

Here's a first pass at adding a composer.json file for this project. You might want to adjust it. It would be great to add this and submit to packagist.org so it's easy for others to find and install this project using Composer.

Case in point, I'd like to use this library for my project (https://github.com/kostajh/ac-task-php) and being able to install via Composer would be great.

Thanks!

Kosta