[DATABASE] Add untouchables property to Model

Allow a model to skip timestamps update if the updated attributes are set as *untouchables*.

**Use case:**
Let's say you have a `user` table, with a `logged_at` column. Your `User` model has `timestamps` enabled : each time a user login you want to track his last connection date. The drawback of that is that the `updated_at` column will also be updated, and it does not really reflects the last time a user has *really* updated his profile.

**Solution:**
Add `logged_at` to the `untouchables` array. This way, if `timestamps` is enabled, and if the only attributes that you change are referenced as `untouchables`, the model will be saved (hopefully) but the `updated_at` attribute will remain the same.