FIX: missing Model::__unset($property)

[qyanu]

I found out, that the Model class wasn't defining the __unset method in order to support unset($model->property).

This commit/patch implements __unset by just calling $this->orm->__unset, as is already done for __set, __get and __isset.

[qyanu]

Better late than never: Observed and followed the guidelines for making a pull request, thus adding a unit-test plus changelog.

[treffynnon]

Merged in develop. Thanks.