Added composer meta-data file, for use with composer/packagist

This change adds a composer.json file, so that the package can be listed at https://packagist.org/

If you're not familiar with Composer/Packagist, it's a PHP package management tool that is gaining a lot of popularity projects such as Symfony, Drupal, and SilverStripe (which I'm involved in).  It differs from PEAR in that the packages are downloaded per-project, rather than being installed in a system location.

The attached composer.json file, if you merged it in and then submitted the repository at https://packagist.org/, would mean that anyone using composer can add this to their project by running 'composer require antonterekhov/orientdb-php', after which the code would be available at ./vendor/antonterekhov/orientdb-php.

I wasn't sure if the package name should be something different, e.g. netmonsters/orientdb-php.  I'm happy to change the pull request if needs be, or of course, you could edit it yourself. :-) 

More information is available at http://getcomposer.org/