Add events page and RSS feed

This pull request adds an Events page under Resources with a RSS feed.
Page can be previewed live here : http://bitcoinevents.us.to/en/events

Events can be added in _events/ using instructions in the README. They are handled by events.rb plugins which makes them available from the site.pages variables. Previous orphan events pages in _post/events are dropped by the same occasion.

Only basic informations are displayed (title, date, link, location) in order to invite visitors to visit each conference website, to stay relevant in translated pages and to remain scalable in the event that conferences start to appear in various countries over time.

Links to existing meetup lists are displayed at the bottom of the page in order to keep focus on more important events with known speakers / attendees or with enough participation.