Field values are lost when pickling jsonmodel

I found out that using pickle serializer does not seem to work with jsonmodels. Using this simple model as an example (using Python 2.7.10):

```python
class TestModel(models.Base):
a = fields.StringField(default="")
```

```python
>>> m = TestModel(a="foo")

>>> m
TestModel(a='foo')

>>> pickle.loads(pickle.dumps(m))
TestModel(a='')
```

I'm using Python 2.7.10.

So the field value is lost when pickling. I found out that pickle uses `__dict__` variable by default, and as this does not contain field data, it is not saved to the serialized form.

Using customized `__getstate__()` and `__setstate__()` methods I managed to make it work, I just had to make sure the `_cache_key` variable is preserved, as unpickling the object does not run `__init__()` and `_cache_key` is not created:

```python
class TestModel(models.Base):
a = fields.StringField(default="")

def __getstate__(self):
state = self.to_struct()
state['_cache_key'] = self._cache_key
return state

def __setstate__(self, data):
self._cache_key = data.pop('_cache_key')
self.populate(**data)
```

Does this seem to be a correct way to handle pickling? Also, is it necessary to pickle the `_cache_key`, or would it be ok to just initialize it to a new value? (Some kind of `_init_cache_key()` method would be a good addition in that case).

If this is a correct direction, I can make a pull request and test it with other Python versions. Support for JSON serializer would also be possible.