Support for complex entity nesting

[stanlemon]

It would be nice/cool if you could deep-nest entities in ng-admin when performing a save operation. As an example, with the demo data if I could embed a full tag object inside of the tags[] when I post, rather than just a collection of the ids for the included objects.

[manuquentin]

Hello,
Nice idea, I'll add an option to send the full object when using a Reference field.

[fzaninotto]

Fixed by #525