Add RSVP.js

https://github.com/tildeio/rsvp.js

As requested in https://github.com/tildeio/rsvp.js/issues/206
Merge pull request #399 from megawac/rsvp

Add RSVP.js
Merge pull request #400 from lewisgoddard/master

Add jQuery.downBoy by eustasy
Merge pull request #2 from jsdelivr/master

bring inline with main