Added setParent method to Mapping

Using Elastica for a small school project again… ;-)

This PR allows you to write 

```
$mapping = new \Elastica\Type\Mapping();
$mapping->setParent('blog');
```

instead of

```
$mapping = new \Elastica\Type\Mapping();
$mapping->setParam('_parent', array('type' => 'blog'));
```

ElasticSearch documentation: http://www.elasticsearch.org/guide/reference/mapping/parent-field/