[WIP] add usecase subscriber

i'm not happy with the naming - do you have any ideas?
I will add unit tests later :wink: 

Example:

```php

class UserUseCase extends UseCaseSubscriber 
{

  public register()
  {
    return [
      'Command\UserEditCommand' => 'edit',
      'Command\UserAddCommand' => 'add'
    ];
  }

  public function edit(UserEditCommand $command)
  {
    // ....
  }

  public function add(UserAddCommand $command)
  {
    // ....
  }
}
```