add spinner when loading

[ByronHsu]

Add spinner when loading.
To make AAG-Visualizer more fancy.
There are two kinds of loading process in my web page.

1. In the beginning of opening the website.
2. Click the visualize button and start visualization.(there may be a delay when the circuit is large, so I hold that we can add a spinner below indicating that it is in the transformation process.)

Help some guy can help implement this new feature. Thanks.