Prismic::Fragments::Group#each

Looping through Group fragments had to be done this way:

```ruby
@document['docchapter.docs'].fragment_list_array.each { |group| do_something }
```

Now it's possible to do it more straight-forwardly this way :

```ruby
@document['docchapter.docs'].each { |group| do_something }
```

Actually, isn't there a way to deport all of the `fragment_list_array` field's methods to the Group object? Group already inherits Fragment, so it's not possible to have it inherit Array...

There's no emergency for now, I think I'm the only person using groups today, and I've simply monkey-patched the only project where I need to do that, for now.