handle the FP front-end JS snippet and provide a custom RSS feed

The tricky part in this was how to handle flushing the rewrite rules. See comments in display_settings_page() for my solution. Doing it on plugin activation won't work, as that happens before the feed is initialized (the flushing has to happen after). I considered having a settings flag for it, but that would mean an extra db call to check it every time the RSS feed is hit, in perpetuity, which seemed inappropriate.

To see the feed, add a post after you activate the plugin and save your API key, and then go to http://your_site/your_wp?feed=versa (or /feed/versa may also work, depending on your permalink structure)