Remove test code from your source

This pull request was made to let the source code clean.
The factory definition should stay out of source code, only on test code.

For this, I created the `define` function, it register a factory for a given model.
Also, the class now accepts Closures as attribute factories.

Example:
```php
$factory->define('SomeModel', array(
    'name' => 'string',
    'somethingelse' => function() {
        return 'just a string';
    }
));
```