Backport the following changes:

Backport the following changes:

Misc/NEWS 1.387->1.388
Lib/test/string_tests.py 1.10->1.11, 1.12->1.14,
Lib/test/test_unicode.py 1.50->1.51, 1.53->1.54, 1.55->1.56
Lib/test/test_string.py 1.15->1.16
Lib/string.py 1.61->1.63
Lib/test/test_userstring.py 1.5->1.6, 1.11, 1.12
Objects/stringobject.c 2.156->2.159
Objects/unicodeobject.c 2.137->2.139
Doc/lib/libstdtypes.tec 1.87->1.88

Add a method zfill to str, unicode and UserString
and change Lib/string.py accordingly
(see SF patch http://www.python.org/sf/536241)

This also adds Guido's fix to test_userstring.py
and the subinstance checks in test_string.py
and test_unicode.py.