Added Django's models.IPAddressField to the ModelSerializer field mapping

Very simple change.  Since IPAddressField is a CharField, Django defines an implied max_length that was not being popped from kwargs by ModelField causing an invalid kwarg passed to Field's __init__:

```
  File "build/bdist.macosx-10.10-intel/egg/rest_framework/fields.py", line 1170, in __init__
  super(ModelField, self).__init__(**kwargs)
  TypeError: __init__() got an unexpected keyword argument 'max_length'
```