Add composer configuration file

Hi,

This pull request add the composer configuration file for easy project dependencies management using composer.

As you can see in the new README, you have to add the repositories configuration in your project composer.json for now because composer looks for the repository matching "flaptor/indextank-php" using packagist information.

Could you register your library in https://packagist.org/ ? After that, you can remove the repositories configuration part in the README because it will not matter anymore.

Thanks and Regards