Add links to Org mode and GNU Emacs websites in creator strings.

* org-html.el (org-export-as-html): Add links to the Org mode
and GNU Emacs websites When :html-postamble is set to 't.

* org-export.el (org-export-creator-string): Add links to the
Org mode and GNU Emacs websites.

contrib/lisp/org-export.el

@@ -354,7 +354,7 @@ If the value is `comment' insert it as a comment."
 	  (const :tag "Insert the sentence" t)))
 
 (defcustom org-export-creator-string
-  (format "Generated by Org mode %s in Emacs %s."
+  (format "Generated by <a href=\"http://orgmode.org\">Org</a> mode %s in <a href=\"http://www.gnu.org/software/emacs/\">Emacs</a> %s."
 	  (if (fboundp 'org-version) (org-version) "(Unknown)")
 	  emacs-version)
   "String to insert at the end of the generated document."

lisp/org-html.el

@@ -1835,7 +1835,8 @@ PUB-DIR is set, use this as the publishing directory."
 			  (split-string email ",+ *")
 			  ", "))
 	      (creator-info
-	       (concat "Org version " (org-version) " with Emacs version "
+	       (concat "<a href=\"http://orgmode.org\">Org</a> version "
+		       (org-version) " with <a href=\"http://www.gnu.org/software/emacs/\">Emacs</a> version "
 		       (number-to-string emacs-major-version))))
 
 	  (when (plist-get opt-plist :html-postamble)