Follow MRO to collect declared fields from parent classes and mixins

Following the pattern of django.forms.forms.DeclaritiveFieldsMetaClass the __new__ method has been changed to follow parent classes and pick up the declared fields. This allows:

    class BaseResourceMixin(object):

        name = fields.Field(
            column_name='Name',
            attribute='name'
            )
        description = fields.Field(
            column_name='Description',
            attribute='description'
            )

    class ThingResource(ResourceMixin, resources.ModelResource):

        user = fields.Field(
            column_name='User',
            attribute='user',
            widget=UserWidget(
                get_model(*settings.AUTH_USER_MODEL.split('.'))
                )
            )

        class Meta:
            model = Thing
            fields = ('name', 'description', 'user')

No tests yet but happily used as is.

Regards,
Darryl
Simple mixin to test that fields are picked up by the child class.