Release as a Composer package

Hi, first of all, thank you for creating this PHP port of YUI CSS compressor. It has come handy.

Getting this thing on [Composer](http://getcomposer.org) would be awesome. This library is one of the few last libraries, that I use, that aren't available though composer.

Composer might seem pretty confusing if you aren't already familiar with it, but fear not; getting a Composer package out there isn't hard. At minimum it only requires the JSON meta file (as in the PR). Adding the meta file is the minimum, but to get most out of Composer, the library should also be registered on the central repository, [Packagist](https://packagist.org/). Registering the package makes consuming it easier, as you can refer to it by its short name (see the slightly altered README.md in the PR).

To make version contraints resolvable, the repository should use [semantic versioning](http://semver.org/), which is what Composer uses. Otherwise users can not define contraints; Users can fetch the library, but only by referencing a commit, a tag, a branch or by a wildcard.

In short, this all comes down to few simple steps;

1. Add composer.json meta file to the repository.
2. Register an account to [Packagist](https://packagist.org/)
3. After you have an account, you can submit the repository to the central index. Click the big *submit package* button, and enter in the link to the [URL](https://github.com/tubalmartin/YUI-CSS-compressor-PHP-port.git) to the repository.
4. After done, you could also add a service hook on GitHub to Packagist. This makes sure Packagist gets informed about any new commits and releases. You can find Packagist service on GitHub, and Packagist key on your Packagist profile page.
5. Create semver tags on releases/milestones (e.g. 2.4.71, 2.4.8, 3.2.6-alpha.1).

Regards,
Jukka