Better Support for non-composer Extensions 

A few changes to the extensions registration to enable quick post-initialize addition of a local extension.
In bootstrap the following will now work....
    ....
    $app->initialize();
    $app['extensions']->register(new My\Extension($app));
    $app->run();
    ...

Edit: Note that this works perfectly fine without it being a git repo, all you need is that your classes can autoload, or you manually require them.