[Security] Add simpler customization options

The goal of this is to provide a simpler extension point for people that don't have the time to dive into the whole security factory + authentication provider + user provider + authentication listener + token mess. As it stands, it gives you a way to just create one class that is handling all the security stuff in one (by implementing SimpleFormAuthenticatorInterface and UserProviderInterface) + one or more token classes.

I would like feedback on whether people think this makes sense or not before continuing and doing a SimpleHttpAuthenticatorInterface for non-form based stuff.

Just FYI that's how it would look in security.yml:

```yaml
security:
    providers:
        simple:
            id: simple_authenticator
    firewalls:
        foo:
            pattern: ^/
            simple_form:
                provider: simple
                authenticator: simple_authenticator
```

/cc @atrauzzi (who posted a long rant on the ML about how hard this all is, and I can't agree more - I hope it's the right account on github?)