Autoloading via composer

PHPStan IMO should not depend on composer autoloader of the project. It is not a direct (dev) dependency and no code depends on it. That's the main difference to other dev tools such as PHPUnit, the code is directly dependent on PHPUnit. 

Would be great to use it as "tool" in CI:

```bash
composer create-project phpstan/phpstan --no-dev
./phpstan/phpstan analyse ./myproject
```