Has one bugfixes

* Always create `hasOne` association field on model instances.<br/>
  No longer need to do `new Person(company_id: null)` if you want to set `company_id` later.
* Specifying `company_id` and saving no longer results in a recursive infinite loop!
* Shell models `Person(4)` are explicitly marked as such.
* `getAccessor` with shell models perform the correct model lookup!
* Exposes `Instance.isPersisted`

It should fix issue #198