Added namespaces and annotations to Cookbook for using ZfcRbac with Doctrine ORM.

The `use` statements should be complete.

Is extending `ZfcUserEntity` correct too?

In my Application I had to add the `Entity` annotations otherwise I got this error:
**Class "Application\Entity\User" sub class of "ZfcUser\Entity\User" is not a valid entity or mapped super class.**

Mentioned in issue https://github.com/ZF-Commons/zfc-rbac/issues/17 .