Allow to override handlers for `Entity` & `Collection`, close #958

Allow to override handlers for `Entity` & `Collection`.

This feature required to move up the itemization step to the model level. Moving this step up will allow more flexibility on how itemization is done. However you can now configure/modify adatpt the item right from a model. this will make easier to do:
- [STI](http://en.wikipedia.org/wiki/Single_Table_Inheritance) (i.e. changing the model name on the fly)
- processing on the datas (date format or adding a virtual field (e.g fullName = lastName + firstName)