Add support for server specific nicknames.

discord/user.py

@@ -125,3 +125,14 @@ class User:
         This is when the user's discord account was created."""
         return snowflake_time(self.id)
 
+    @property
+    def display_name(self):
+        """Returns the user's display name.
+
+        For regular users this is just their username, but
+        if they have a server specific nickname then that
+        is returned instead.
+        """
+        nickname = getattr(self, 'nick', None)
+        return nickname if nickname is not None else self.name
+