Add Composer support to FirePHP Core

[Composer](http://getcomposer.org/) is a dependency management system for PHP packages. This pull request adds Composer support to FirePHP Core. Having this means that a project that depends on FirePHP could put this in their composer.json file to have it automatically download and autoload FirePHP:

    {
        "require": {
            "firephp/firephp-core": "*"
        }
    }

When installing the client application, you would run "php composer.phar install", and it would download FirePHP-Core, and have the FirePHP class autoloaded so that the files would be included automatically when the FirePHP class is used. Any package on http://packagist.org could then also depict a dependency on the package if needed.

Thanks a lot!