PEP 308 implementation, including minor refdocs and some testcases. It

breaks the parser module, because it adds the if/else construct as well as
two new grammar rules for backward compatibility. If no one else fixes
parsermodule, I guess I'll go ahead and fix it later this week.

The TeX code was checked with texcheck.py, but not rendered. There is
actually a slight incompatibility:

>>> (x for x in lambda:0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: iteration over non-sequence

changes into

>>> (x for x in lambda: 0)
  File "<stdin>", line 1
    (x for x in lambda: 0)
                     ^
SyntaxError: invalid syntax

Since there's no way the former version can be useful, it's probably a
bugfix ;)

Parser/Python.asdl

@@ -52,6 +52,7 @@ module Python
 	     | BinOp(expr left, operator op, expr right)
 	     | UnaryOp(unaryop op, expr operand)
 	     | Lambda(arguments args, expr body)
+	     | IfExp(expr test, expr body, expr orelse)
 	     | Dict(expr* keys, expr* values)
 	     | ListComp(expr elt, comprehension* generators)
 	     | GeneratorExp(expr elt, comprehension* generators)