Add property_fields attribute for fast model property access

I was quite dependent on pistons property lookup, so I built this, fields in property_fields will have fields generated for them automatically.

e.g:

```python
class Spling(serializers.ModelSerializer):
    class Meta:
        model = Post
        fields = ('id', 'title', 'content', 'get_absolute_url', 'author_denormalized')
        property_fields = ('get_absolute_url', 'author_denormalized')
```
where get_absolute_url and author_denormalized are properties on the model

thanks! really digging the project