Overhaul admin UI

[justintadlock]

Version 0.1.0 of this plugin was just a proof-of-concept. Eventually, I'd like for the "Whistles" and "Whistle Groups" admin screens to be integrated into a single screen. I'm thinking something along the lines of building off what WP nav menus are doing.

I haven't really looked at what's possible with core as far as re-using code goes though. I certainly wouldn't want to build and maintain a custom screen. If core's not ready, I want to hold off.