Add support to look up existing entities by composite

see https://github.com/ddeboer/data-import/issues/115
Why not try to find the entity by multiple fields? Like:

```php
$repository->findOneBy(
    array(
         $compositeKey[0] => $item[$compositeKey[0]],
         $compositeKey[1] => $item[$compositeKey[1]],
         // etc.
    )
);
```