Added a DoctrineTokenProvider in Security/Core/Authentication/RememberMe

Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Symfony2 tests pass: yes
Todo: Add tests and documentation
License of the code: MIT

In the directory Symfony/Component/Security/Core/Authentication/RememberMe there is only one TokenProvider that can be used, and that is the InMemoryTokenProvider.
I think it should be an example DoctrineTokenProvider there as well, that gives an example of how to store the tokens in the database.