A composer installable library & a symfony/console cli app

* App is reorganized into the class `\WPReadme2Markdown\Converter` with static method `convert($readme, $pluginSlug = null)`
* Minimum PHP version to run is 5.3.3 now (composer and symfony requirement)
* Can be installed by composer as `wpreadme2markdown/wpreadme2markdown`
* Can be compiled into a single file `wp2md.phar`
* New online version as a separate project: https://github.com/sunchaserinfo/wpreadme2markdown.com (PHP 5.5, Slim, Bootstrap, Markdown preview by Parsedown)