Registry::extract() method and a test for it

New method to extract a portion if data as new Registry instance.
Example of use:

    $config = new Registry();
    $config->loadFile('really-huge-json-config-for-all-systems');
    //Routines
    $rendererConfig = $config->extract('renderer');
    $rendererConfig->set('templates.dir', '/path/to/templates');
    $renrerer = new Renderer($rendererConfig);