API: /events v3

[rdohms]

Implement the new functionality for v3:

• http://www.meetup.com/meetup_api/docs/:urlname/events/:id/attendance/#list
• http://www.meetup.com/meetup_api/docs/:urlname/events/:id/payments/
• http://www.meetup.com/meetup_api/docs/:urlname/events/:id/watchlist/#create