link persona in the footer. closes mozilla/123done#12

packages/123done/static/index.html

@@ -38,7 +38,7 @@
     <p>Your task list is stored on the computer for you to use. If you want to sync 123done across devices, log in with Mozilla Persona.</p>
   </section>
   <footer>
-    <p><strong>123done</strong> is a project to show the power of Persona. Written by the identity folks at Mozilla.</p>
+    <p><strong>123done</strong> is a project to show the power of <a href="https://browserid.org">Persona</a>. Written by the identity folks at Mozilla.</p>
   </footer>
   </div>
   <script src="zepto.min.js"></script>