org-journal-mode is now called with customized org-journal-dir

Whenever org-journal-dir or org-journal-file-pattern are updated,
auto-mode-alist will be updated, too. Before this, if org-journal was
required before org-journal-dir or org-journal-file-pattern were
customized, it would register itself for the wrong files in
auto-mode-alist.
calendar should mark entries even when org-journal was not started yet

Added an autoload form that loads org-journal when the calendar is invoked.