Remove use of request info to identify user in AuthComponent::login().

Your login action would now be similar to:
```php
...
if ($this->request->is('post')) {
    $user = $this->Auth->identify();
    if ($user) {
        $this->Auth->login($user);
        // redirect somewhere
    }
    // set error flash
}
...
```
> It doesn't really have to be an array, does it?

The way `AuthComponent::user()` works it has to be an array.

> I think this will still confuse people, as most expect that by passing data it will run the authentication routine instead of just storing the data.

Agreed. dereuromark suggested `update()` but it doesn't feel right. More suggestions for alternate name are welcome.