Improved mobile experience

[rspeed]

I've made a few changes with the intent of making this utility work a bit better on mobile devices.

1. There's now a manifest file, which tells browsers to store the whole app locally. That makes it load immediately, and also makes it work offline. The main caveat of this, however, is that you'll need to bump the "build" number (in the manifest file) when other changes are made. Otherwise browsers won't update cached resources.
2. I added an icon for when users add it to their phone/tablet as an icon. It's a really stupid-looking porkchop. I'm not an artist.
3. Made a small addition to the CSS so that form elements don't butt up to the edge of the screen.