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

This allows to create a bi-directional association from a custom
implemation of class User to UserRegistration.

According to:

http://stackoverflow.com/questions/17588682/doctrine-inheritance-replacement
Merge pull request #1 from FranzBruckner/master

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