Custom datetime fields on models

This will allow you to easily set which fields should be converted to/from Carbon objects when setting/getting their values, just like created_at, updated_at and deleted_at - without having to write custom accessor/mutator methods for each of them. Example:

```php
// example model class
class Article extends Eloquent
{
    protected $dateTimeFields = array('pub_time');
}

// somewhere else...
$article = Article::first();
echo $article->pub_time->format('j. F H:i');
```