Bug fix for timestamp-whole-year difference

Implement support for ISO week dates

New %timestamp-decode-iso-week function and supporting
FORMAT-TIMESTRING changes, including an +iso-week-format+ constant.

Fix for format-timestring's destination argument

Make FORMAT-TIMESTRING write to *STANDARD-OUTPUT* when T is passed as
DESTINATION argument, like promised in the docstring.  WRITE-STRING
accepts a stream designator as destination, and T as a stream
designator designates *TERMINAL-IO*.

*default-timezone* was used before being defined, causing a compile warning.

 in format-rfc3339-timestring omit-timezon-part defaults to omit-time-part, because you most probably don't need the timezone if the time itself is not displayed.

useful things for copy/pasting:
darcs get http://common-lisp.net/project/alexandria/darcs/alexandria/
darcs get http://dwim.hu/darcs/hu.dwim.stefil
(asdf:test-system :local-time)

 roll back my change where adjust-timestamp defaulted to UTC (both are broken, but this way it's at least backwards compatible)