+ Changed org-export-directory to +org-export-dir (conform to naming
convention). It turns out org-export-directory never existed in org.
+ Make org-export-backends addition (for ox-pandoc) idempotent.
+ Fix redundant forward slash in org-publish-timestamp-directory.
+ Resolve export directory later, giving the user a larger window to
change +org-export-dir.
I prefer not to invent new variables when they aren't strictly
necessary. org-directory is one such variable (although the other path
variables are still necessary).
lang/org's initialization process is now split up into hooks on
org-load-hook. This approach is cleaner and easier to customize. I also
removed the escape binding in org-agenda-mode-map, as the popup system
makes it redundant.
