ox-beamer: Fix undefined reference with internal links

* lisp/ox-beamer.el (org-beamer-target): Use label macro instead of
  hypertarget.

Moreover, target syntax cannot take advantage of hypertarget's second
parameter.

<http://permalink.gmane.org/gmane.emacs.orgmode/92455>

lisp/ox-beamer.el

@@ -777,7 +777,7 @@ contextual information."
   "Transcode a TARGET object into Beamer code.
 CONTENTS is nil.  INFO is a plist holding contextual
 information."
-  (format "\\hypertarget{%s}{}"
+  (format "\\label{%s}"
 	  (org-export-solidify-link-text (org-element-property :value target))))