Backport of PEP 3101, Advanced String Formatting, from py3k.

Highlights:
 - Adding PyObject_Format.
 - Adding string.Format class.
 - Adding __format__ for str, unicode, int, long, float, datetime.
 - Adding builtin format.
 - Adding ''.format and u''.format.
 - str/unicode fixups for formatters.

The files in Objects/stringlib that implement PEP 3101 (stringdefs.h,
unicodedefs.h, formatter.h, string_format.h) are identical in trunk
and py3k.  Any changes from here on should be made to trunk, and
changes will propogate to py3k).

Lib/test/test_descrtut.py

@@ -183,6 +183,7 @@ Instead, you can get the same information from the list type:
      '__delslice__',
      '__doc__',
      '__eq__',
+     '__format__',
      '__ge__',
      '__getattribute__',
      '__getitem__',