Add StringToObjectConverter

This PR added the abilitly to convert a string to a object.

Use case:
You have a CSV file and want to import the CSV file into your database. Now you can map this property, with this converter.

```php
$workflow = new Workflow(new CSVReader($file));
$workflow->addValueConverter('name', new StringToObjectConveter($repository, 
'username'));
$workflow->addWriter(new DoctrineWriter($objectManager));
$workflow->process();
```