Added PotionRestView Function and updated urls.py

PotionRestView and CreatureRestView, and updated urls.py
You'll also need to add one for the collection as a whole, and should give them names:
```
url(r'^api/potions/(?P<id>\d+)/$', views.PotionRestView(),  name='potion_api')
url(r'^api/potions/$', views.CreatureRestView(),  name='potions_api')
```