Multi level array value converter

Add an ArrayValueConverterMap (fix for https://github.com/ddeboer/data-import/issues/22).

Example Usage:

```php
$converter = new ArrayValueConverterMap(array(
    'name' => new SomeValueConverter()
));

$data = array(
  array('name' => 'foo'),
  array('name' => 'bar'),
);

$converter->convert($data);
```