set metadata for interface to be able to fetch entites by interface name

using the new ResolveTargetEntity functionality we noticed we needed another feature:

From the Symfony Bundle defining the interface, we'd like to be able to fetch entities by this very interface name, e.g.:

``` php
$em->find('Foo\BarBundle\Entity\PersonInterface', 1);
```

or

``` php
$em->getRepository('Foo\BarBundle\Entity\PersonInterface')->findAll();
```

This PR sets metadata for the interface when metadata for a class is loaded that the interface is configured for