Fix speedkeys to move subtrees up and down

* lisp/org.el (org-speed-commands-default): Bind "U" and "D" to
`org-metaup' and `org-metadown'.

Org speed keys were not updated to reflect the recent changes to
org-shiftmetaup and org-shiftmetadown.  As a result the default
bindings for speedkeys "U" and "D" were destructive, since they moved
only single lines.  Binding them to org-metaup and org-metadown fixes
the problem.

Thanks to Marco Wahl for reporting the problem:
http://permalink.gmane.org/gmane.emacs.orgmode/87873

lisp/org.el

@@ -19523,8 +19523,8 @@ boundaries."
     ("s" . org-narrow-to-subtree)
     ("=" . org-columns)
     ("Outline Structure Editing")
-    ("U" . org-shiftmetaup)
+    ("U" . org-metaup)
-    ("D" . org-shiftmetadown)
+    ("D" . org-metadown)
     ("r" . org-metaright)
     ("l" . org-metaleft)
     ("R" . org-shiftmetaright)