objtype: super: Fall back to "object" lookup as last resort.

Also, define object.__init__() (semantically empty, purely CPython compat
measure). Addresses #520.

tests/basics/class_super_object.py

@@ -0,0 +1,15 @@
+# Calling object.__init__() via super().__init__
+
+class Test(object):
+    def __init__(self):
+        super().__init__()
+        print("Test.__init__")
+
+t = Test()
+
+class Test2:
+    def __init__(self):
+        super().__init__()
+        print("Test2.__init__")
+
+t = Test2()