modify getPassword so it respect getPasswordName

when trying to extend this model, and using custom password field, it not respect overrided getPasswordName() 

e.g. I want to change password field to other field
```php
class ProviderCredential extends Cartalyst\Sentry\Users\Eloquent\User {

    public function getPasswordName()
    {
        return 'OtherThanDefaultPasswordField';
    }
}
```
Merge pull request #444 from xsanisty/master

modify getPassword so it respect getPasswordName