Allow a prefix for model class names

This patch will check to see if there is a class_prefix in ORMWrapper::$_config and if there is prepend it to the class_name.

This allows creating model classes less likey to clash with existing classes but keep the factory method simple:

```php
// Configure a class prefix (defaults to empty)
ORMWrapper::configure('class_prefix', 'Model');

// Create entity
class ModelUser extends Model {}

// Create user
$user = Model::factory('User')->create();
```

Thanks