chore(composer): Rely on composer autoloader to load core classes

If we included it, we could
- give nicer message if autoloader file does not exist (sth like: "You need to run composer.phar install to use this project")
- register autoloading from plugins as well

See example:
```
$loader = require 'vendor/autoload.php';
$loader->add('Acme\\Test\\', __DIR__);
```