Composer support

This PR adds support for installing jPlayer via [composer](https://getcomposer.org/) to simply install and use it and keep it updated in Symfony2 applications and other projects that use composer.
Also added a short installation guide for now. If you merge this, you might want to register jPlayer on [Packagist](https://packagist.org/) to get rid of the `"repositories": {}` part that is currently needed to install it.