add requestAnimationFrame

[jeroensak]

add requestAnimationFrame to optimize smooth animations. Also fallback for browsers without support.

[dbramwell]

Hi,

Thanks for the pull request. I might be missing something, as I hadn't heard of requestAnimationFrame before now, but I'm not sure that this is how it's supposed to be used. From the documentation I think it's supposed to be used to actually change the style of an element, similar to this example. React-animate-on-scroll uses CSS animations, so it doesn't work in the same way. I'm very possibly missing something though. Could you explain how the 'requestAnimationFrame' function helps?

Also, the tests seem to have broken, these would need to be fixed before merging.