handle events and backfeed rsvps

[snarfed]

only for FB and G+ right now, since they're the only ones that do events.

mostly just consists of adding event and rsvp support to activitystreams-unofficial. after that, bridgy should automatically do original post discovery, send webmentions for rsvps, and render the rsvps as mf2 without any additional bridgy-specific code.

thanks to @aaronpk and @tantek for the idea!

[snarfed]

looks like G+ events are exposed as Moments. they're exposed normally in FB's graph api, natch.

[aaronpk]

Here are example URLs for you:

• http://aaronparecki.com/events/2014/01/29/1/homebrew-website-club
• https://www.facebook.com/events/1453049941575679/

[snarfed]

done for facebook. example: http://aaronparecki.com/events/2014/01/29/1/homebrew-website-club

[snarfed]

i was wrong, the G+ API doesn't expose events after all. when someone shares an event, you can see it in their stream with activities.list and get, but that's just a note about it, not the event itself. here's the G+ API bug asking for events: http://code.google.com/p/google-plus-platform/issues/detail?id=567