Reformed horiz_nav_bar, added pop-over layer with preliminary styling,

moved login inputs to hidden pop-over layer, added onclick for btn_login

src/interface/static/js/pyshelf_ux.js

@@ -71,4 +71,7 @@ $(document).ready(function(){
            $(this).attr("value", p_string);
         }
     });
+    $('#btn_login').on('click', function(){
+        $('#hdr_nav_login').toggle();
+    });
 });