Display meetups on the events map

Live preview: http://bitcoinmeetup.us.to/en/events

As suggested in issue #326, #318, #325 , this pull request uses the Meetup API to display all meetups on the map on bitcoin.org, giving extra visibility to all of these events.

The events.rb plugin should be error-proof; the website builds correctly even if the API call fails or is returning inconsistent data.

Some small improvements are also part of the pull req, like not displaying past events even if they are still in _events/ and using global liquid variables instead of generating empty orphan event pages.