Cleanup referenced GridFS files when a document is deleted

Note that drop_collection is not modified since there is no
guarantee that a GridFS collection holds files for only one
Document class.  Otherwise you could drop files for other fields
or documents accidentally.

mongoengine/document.py

@@ -277,6 +277,11 @@ class Document(BaseDocument):
         signals.pre_delete.send(self.__class__, document=self)
 
         try:
+            for field_name in self._meta['proxy_fields']:
+                proxy_class = self._meta['proxy_fields'][field_name]
+                if hasattr(proxy_class, 'delete'):
+                    proxy = getattr(self, field_name)
+                    proxy.delete()
             self.__class__.objects(pk=self.pk).delete(safe=safe)
         except pymongo.errors.OperationFailure, err:
             message = u'Could not delete document (%s)' % err.message
@@ -341,6 +346,13 @@ class Document(BaseDocument):
         """
         cls._meta['delete_rules'][(document_cls, field_name)] = rule
 
+    @classmethod
+    def register_proxy_field(cls, field_name, proxy_class):
+        """This method registers fields with proxy classes to delete them when
+        removing this object.
+        """
+        cls._meta['proxy_fields'][field_name] = proxy_class
+
     @classmethod
     def drop_collection(cls):
         """Drops the entire collection associated with this