Message2680

This patch fixes:

o update str and unicode gexpose definitions for str.decode to return a unicode (instead of str) and unicode.encode to return a str (instead of unicode)
o overload PyString's __str__() method in PyUnicode to return a PyString instead of PyUnicode. Fixes str(u"") to return a str instead of unicode

I've also added/uncommented out a couple tests to exercise these changes

The patch is against r3142 of the 2.3 branch. This should probably be merged over to 2.2