Get rid of get_pk, and raise DoesNotExist when needed, instead of ValueError

- Get rid of the `get_pk` method: use `myinstance.pk.get()` instead (as `pk` is a field, it's a consistent way to do it) [close #60]
- raise `DoesNotExist`: in `RedisModel.__init__`, when called with a single argument used as a PK, and this PK was not found, a `ValueError` was raised, instead of a `DoesNotExist` [close #65]