Move up the itemization step to the model level.

Moving this step up will allow more flexibility on how itemization is done. You can now configure/modify adatpt the item right from a model.

Use case:
- [STI](http://en.wikipedia.org/wiki/Single_Table_Inheritance) (i.e. changing the model name on the fly)
- processing the datas (date format or adding a virtual field (e.g fullName = lastName + firstName)
- first step to solve #958, shouldn't be harder to now pass some custom handlers for `Document`.