Changed target-entity of UserRegistration->user from Implemation to Interface

Hi,
i encountered a problem whlie trying do create a bi-directional accociation pointing from my user-class to \HtUserRegistrationDoctrineORM\Entity\UserRegistration.

That was the property in my UserClass:

```php
    /**
     * 
     * @ORM\OneToOne(targetEntity="\HtUserRegistrationDoctrineORM\Entity\UserRegistration", mappedBy="user")
     * @var \HtUserRegistration\Entity\UserRegistrationInterface
     */
    protected $mailVerification;
```

However, doctrine refused to work with this configuration, since your were pointing to ZfcUser\Entity\User
```xml
<one-to-one field="user" target-entity="ZfcUser\Entity\User">
```
According to the post on stackoverflow i did some changes. http://stackoverflow.com/questions/17588682/doctrine-inheritance-replacement

Maybe you want to merge the changes.

Further enhancement would be having an Interface for Entity\UserRegistration.

Best