BugFix of mongoDb support

It's a fix of severe bug.

Support of mongoDB by Sonata-User-Bundle was completely broken, because Doctrine was unable to find `FOS\UserBundle\Resources\config\doctrine\User.mongodb.xml` while it iterate through inheritance.

So basically FOSUserBundle has `FOS\Model` which is extended twice by `FOS\Entity` and by `FOS\Document`.

At the same time Sonata had similar logic, but Sonata\UserBundle\Model\User has extended `FOS\Entity`.
It really doesn't make any sense. The only reason of existence of this Model is preventing from copy-pasting common field between Entity and Document, but the price of it is too high.

Since support of PHP 5.3 is necessary using of traits is impossible, so I just copied methods from Entity into Document.
Also I've fixed logic of `UserManagerInterface` and `UserManagerProxy` which caused similar problem with serializer. So basically missed declaration of service for mongoDB resulted in Fatal Error on project update.
By now all methods of newly updated `UserManagerInterface` for `Document\UserManager` is replaced with stubs throwing an Exception. I suppose serialization wouldn't work for mongo, but looks like everything else is up an running.