Fixing auto purge cron that was not being created properly

Fixes #412

* Using `wp_loaded` hook instead of `wp`
* Clear out all cron events when plugin is uninstalled
* Clear out deprecated cron event that is lingering around
* Clear out new cron event when plugin is updated so new schedule takes effect immediately

I don't think there is anything wrong with running this hook on every page load since nothing happens if the event is already scheduled. It seems like a pretty lightweight operation to me.

@lukecarbis please review and merge if satisfied.