Revamp format_date() to work with PHP5 timezones (if available)

This goes with the patch at http://drupal.org/node/11077, which creates the date_handle_timezones() function and other necessary infrastructure.

The reason for creating date_handle_timezones() is that we can tweak it as we figure out exactly what versions we require to do what we want to do. I would keep the document generic, i.e. 'Using native timezone handling mode' instead of naming a specific version.

Also, I see you omitted the 'r' format from the native timezone handling code and added it to the non-translated dates. You have to run that one back through format_date() as is done with the offset handling because there are parts of the format that need translating.

Have you done any testing to see if the dates are correctly computed?