RFC: HABTM support

Hi guys, I started the lithium adventure one year ago. Li3 is an awesome framework which I began to be pretty comfortable with. I really want the upcoming 1.0 brings an army of new fans and contributors to make this framework even more popular.

### Why I'm talking about HABTM relations ?

HABTM is considered as a bad abstraction and I'm pretty agree. It's not the best abstraction ever made. However it's an abstraction which it used by numbers of coder as a plinth to higher abstraction (e.g to allow nested saving directly from form submitted datas).

Each time I needed an HABTM associations in projects, it always resulted into a dillution of some copy/paste of extra (read crappy here) code.

Each time I tried to convince friends to start using Li3, they always turned back when I failed to provide a solid alternative for HABTM relation.

### What am I aiming at ?

I'm not afraid to to justify a position even alone against the world, but for the HABTM:
- Li3 does not offer a solid alternative.
- Coder don't like changing their shopping patterns.
- The whole world is against us.

### Conclusion

To conclude, I wanted to show how difficult is it for a newcomer to reach the abstraction provided by a lot of framework like the simple `saveAll` a la CakePHP.

Almost all this PR for allowing the following syntax:
```php
$image = Images::create(array(
	'image' => 'someimage.png',
	'title' => 'Image1 Title',
	'tags' => array(1, 3, 6) // <- This format is compatible with a submited form select
));
$image->save(null, array('with' => 'Tags')); // The image is saved with its associated tags
```

Wanted to know your point of view on this subject and what alternative we can bring to make this kind of abstraction easier to reach for newcomer.

(Don't pay attention to 3ae0bfb && 11e8c1d, It's bugfix I started before going berserk)