Improve RSS support

[pageer]

In the process of investigating export to WordPress, I discovered that the RSS support is severely lacking. We don't even include dates on the entries, which cause WordPress to mark the entries as created January 1, 1970!

To improve things, we should:

1. Just kill off the RSS1 plugin. I don't think people really use that format anymore anyway.
2. Clean up the RSS2 plugin to make sure it's up to spec.
3. Add in support for Atom feeds, which seem to be the de facto standard these days.