add ifvisiblejs [author]

ifvisible.js is a visibility api wrapper that lets you to check if user is looking at the page or interacting with it in a crossbrowser and lightweight way.

ifvisible.js has **956** stars, **51** watchers and **79** forks so it's fairly popular.

I got the files from the [repository itself](https://github.com/serkanyersen/ifvisible.js), I created the minified version myself and run the tests against it.