Fixed custom user class bug with UserProvider

If you copy the User model and create a new one like `class Person implements UserInterface` it doesnt work with the current UserProvider.

I rewrited the whole User class and implemented the UserInterface.
The difference is that my table contains not so much informations like the base user class.
I rewrited the UserManager so that the methods `findUserByEmail`, `findUserByUsername` and `findUserByConfirmationToken` work.
But after a login i got the message that `There is no user provider for user...`.

The problem is that it would work fine if this commit would be accepted.

I could rewrite the UserProvider, too. But why? The UserProvider is nice and usable for me except the part with the direct UserModel check...

Maybe you could add a example how to override the doctrine mapping completly.
I could extend your BaseUser but i cant use your mapping...
If i copy the User.orm.xml from `/vendor/friendsofsymfony/user-bundle/Resources/config/doctrine/model/User.orm.xml` to `/src/Acme/UserBundle/Resources/config/doctrine/model/User.orm.xml` it doesnt work... The bundles parent is `FOSUserBundle`.

Your idea with `Doctrine Inheritance Mapping` doesnt work, too. I dont know how to do?
Your BaseUser is a mapped super class... And i cant remove columns with that inheritance ...

Maybe there is another solution... But i would keep the UserProvider from you instead override them.