Convention over configuration

I think would be better to follow some conventions in order to keep the models cleaner. 
The conventions I propose are taken by the configuration in the documentation:

```php
// Translation model
// example: PostTranslation
get_called_class().'Translation';

// Locale field
'lang';

// Relationship field
// example: post_id 
snake_case(get_called_class()).'_id'; // same as laravel relationship convention
```

I also propose the usage of a new public variable `static::$translatables` used in case we wish to use the above conventions. In this case, `$translatables` is an array containing the names of the fields.

Finally, note the array casting in the following lines. This allows us to use a string instead of an array in the configuration.

```php
private function getTranslatableFields()
{
    return isset(static::$translatable['translatables'])
        ? (array) static::$translatable['translatables']
        : (array) static::$translatables;
}

// Configuration in model
public static $translatables = 'title';

// Or
public static $translatable = array(
    'translatables' => 'title'
);
```