Implements support for handling inverse relationships with toMany attrib...

...ute

This is already handle if we get the toMany attribute of a record and
use pushObject. Like this: bar1.get('fooToMany').pushObject(foo2);

We this commit, the inverse relationships is also notify if we directly
set the attribute with an array. Like this:
bar1.set('fooToMany', [foo2, foo3]);