Turn MediaComment's author() into get_author property

1) MediaComment's author method conflicts with the author
   field. So rename it to get_author.
2) Turn it from a normal function into a python property.
   That means you call it by ".get_author" not by
   ".get_author()". This is exactly what sqlalchemy gives
   us free of charge.

mediagoblin/db/mongo/models.py

@@ -346,7 +346,8 @@ class MediaComment(Document):
     def media_entry(self):
         return self.db.MediaEntry.find_one({'_id': self['media_entry']})
 
-    def author(self):
+    @property
+    def get_author(self):
         return self.db.User.find_one({'_id': self['author']})