New dformat() function for fuzzy date support

Ignore-this: 7193cd788fee2c05a9068bf6edd5dc17

It is now possible to use the %f placeholder in $conf['dformat']
to add a fuzzy age string.

Template developers and plugin authors should replace their strftime +
$conf['dformat'] calls with calls to the new dformat() function.

Example:

   %Y/%m/%d %H:%M (%f) produces dates like this:

   2009/09/16 10:36 (3 weeks ago)

darcs-hash:20091007133614-6e07b-677108d1b43928ef8fd886813e43514507b5e073.gz