Support includeDate config

This was added to jsdoc in jsdoc/jsdoc#916
the feature is described here:
http://usejsdoc.org/about-configuring-default-template.html#showing-the-current-date-in-the-page-footer

Closes davidshimjs#46.

tmpl/layout.tmpl

@@ -63,7 +63,7 @@
         <?js } ?>
 
         <footer>
-            Documentation generated by <a href="https://github.com/jsdoc3/jsdoc">JSDoc <?js= env.version.number ?></a> on <?js= (new Date()) ?>
+            Documentation generated by <a href="https://github.com/jsdoc3/jsdoc">JSDoc <?js= env.version.number ?></a><?js if(env.conf.templates && env.conf.templates.default && env.conf.templates.default.includeDate !== false) { ?> on <?js= (new Date()) ?><?js } ?>
         </footer>
     </div>
 </div>