[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

namespace Example\UseCase;

use APL\UseCaseCollection;
use Example\Command\UserEditCommand;
use Example\Command\UserAddCommand;

class UserUseCase extends UseCaseCollection
{

  public register()
  {
    return [
      'Example\Command\UserEditCommand' => 'edit',
      'Example\Command\UserAddCommand' => 'add'
    ];
  }

  public function edit(UserEditCommand $command)
  {
    // ....
  }

  public function add(UserAddCommand $command)
  {
    // ....
  }
}
```

since php5.5 sugar syntax :wink: 

```php

namespace Example\UseCase;

use APL\UseCaseCollection;
use Example\Command\UserEditCommand;
use Example\Command\UserAddCommand;

class UserUseCase extends UseCaseCollection
{

  public register()
  {
    return [
      UserEditCommand::class => 'edit',
      UserAddCommand::class => 'add'
    ];
  }

  public function edit(UserEditCommand $command)
  {
    // ....
  }

  public function add(UserAddCommand $command)
  {
    // ....
  }
}
```
anyway, i would prefer an other more meaningful naming
e.g. duplicatedCommand and alreadyRegisteredCommand