field of any object type // null

Hi,

thanks for this nice lib.

**Question 1**: Am I correct in assuming that to create a field for any type, I'll have to subclass `fields.BaseField`? See `AnyField` below.

```python
from jsonmodels import models, fields, validators
import json

class AnyField(fields.BaseField):

"""Any field."""

types = (object,)

def parse_value(self, value):
"""Cast value to `bool`."""
parsed = super(AnyField, self).parse_value(value)
return parsed

class Whatever(models.Base):
whatever = AnyField(default=None)

w = Whatever(**json.loads('{"whatever" : ["a", "b", "c"]}'))
print(json.dumps(w.to_struct()))

w = Whatever(**json.loads('{"whatever" : "asdf"}'))
print(json.dumps(w.to_struct()))

w = Whatever(**json.loads('{"whatever" : "asdf"}'))
print(json.dumps(w.to_struct()))

w = Whatever(**json.loads('{"whatever" : null}'))
print(json.dumps(w.to_struct())) # XXX

w = Whatever()
print(json.dumps(w.to_struct()))
```

**Question 2**: Please see line marked with `# XXX` above. Is there any way
to get that printed as `{"whatever" : null}` ? What would I need to change? (Are there any settings I missed?)

Basically I want it to print the same as this...
```python
import json
print(json.dumps({'whatever' : None}))

# prints: {"whatever": null}
```
Or do you reserve None as a special marker? (If so, could that marker be changed
to `class NoneMarker: pass`. But what would that mean for compatibility and upgrades, for other people...?)

Thanks. Regards,
user706