add loading animation

Create _loader.scss

Update main.scss

Update _base.scss

Update gui.js

Update index.html

removed trailing spaces

removed trailing white spaces for real

scss/_base.scss

@@ -28,3 +28,7 @@ h5 {
 h6 {
   font-size: 0.67em;
 }
+
+.hide {
+  display: none;
+}