split iterators-and-generators into 2 chapters, added stubs for introduction to python classes (in iterators chapter)

--HG--
rename : iterators-and-generators.html => generators.html

examples/fibonacci2.py

@@ -1,6 +1,6 @@
 """Fibonacci iterator"""
 
-class fib:
+class Fib:
     def __init__(self, max):
         self.max = max