Add @Property ObjectId support

This will allow for use of `@property` fields to be used for the `ObjectId`

Author: Quinn Chaffee

algoliasearch_django/models.py

@@ -125,7 +125,7 @@ def __init__(self, model, client, settings):
                                                             self.fields)))
 
         # Check custom_objectID
-        if self.custom_objectID in chain(['pk'], all_model_fields):
+        if self.custom_objectID in chain(['pk'], all_model_fields) or hasattr(self, self.custom_objectID):
             self.objectID = get_model_attr(self.custom_objectID)
         else:
             raise AlgoliaIndexError('{} is not a model field of {}'.format(