Add a GenericForeignKeyField

This is a WIP first pass at adding a GenericForeignKeyField. Currently just the dehydrate method is implemented, but it works!

Feedback appreciated.

Here's what the implementation of a Resource using this field looks like:
```
class ItemResource(ModelResource):
    class Meta(ModelResource.Meta):
        model = Item

    class Fields:
        content_object = GenericForeignKeyField(
            attribute='object',
            resource_cls={
                'Bookmark': 'api.views.BookmarkResource'
            })
```
Each time an item with a GFK is dehydrating, we need to use a possibly different resource class, but we are using the same instance of GenericForeignKeyField and hence the same self.resource_cls. So the second time it tries to perform https://github.com/akoumjian/django-conduit/pull/22/files#diff-22a6b820fd297b39e70e7e72af33e7baR328 it will fail because instead of a dictionary it is getting a class.