4 Improvements

[caseywatts]

This tool is just what I've been looking for! :D
I'd been using http://graphviz-dev.appspot.com/ for a while which is a similar idea, but it was buggy and was finally taken down.

Here are the changes in this PR:

• use a two-column layout, and make sure the image doesn't overflow (if the single-column layout is better for some situations, let's discuss what those are?)
• move default example DOT text from the template to the js file, and cache data in the browser's localStorage
• make polling time 500ms instead of 1000ms (I kept getting the diagram rendered with a trailing letter or two missing)
• document the README.md more clearly

I realize PRing a few changes in one isn't best - let me know if you like the full set or if you'd rather I just extract some parts of this.

[yushi]

Thanks for your interest! I'll take a look this weekend.

[caseywatts]

thanks @yushi ! :)

[yushi]

Very nice PR. Thank you for many improvements.
I'll release 0.2.0 to npm soon.