New patch - pep362.7.patch

Summary:

1. Signature & Parameter objects are now immutable

2. Signature.replace() and Parameter.replace()

3. Signature has a new default constructor, which
accepts parameters list and a return_annotation; and
a new 'from_function', which create a Signature object
for the passed function.

4. Parameter.__str__

The implementation has 100% test coverage and is stable.
Please review.

Thanks!