Added sync:import:command

Maybe this looks better, and more simpler:

```
private function getNewChunk()
{
    $this->currentChunk = $this->panther->getChunk(1, $this->documentType, $this->shopId);

    if (empty($this->currentChunk)) {
        $this->valid = false;
        return;
    }

    $this->currentEntity = $this
        ->entityManager
        ->getRepository($this->entityClass)->find($this->currentChunk[0]['document_id']);

    if (!empty($this->currentEntity) || $this->currentChunk[0]['type'] == PantherInterface::OPERATION_DELETE) {
        $this->valid = true;
        return;
    }

    $this->valid = false;
}
```