Detection for buggy raf implementation

Currently, the script performs feature detection by executing raf once and if it returns assuming that implementation is not buggy.

However, there are currently 2 issues with raf on iOS 6 + 7 (tested with both):
* raf doesn't execute at all, which this script can currently handle
* raf executes only after an ongoing animation/transition on the page has finished, which means that if the animation is infinite or takes some time or the transition takes some time, raf will be delayed as long as the transition/animation takes place or won't be executed at all if animation is an infinite one.

We have currently exactly this behaviour, we try to transition some elements while we are dragging another one. The drag is done via raf and element position change handler is not called as long as transition takes place. Tested with iPad2/iOS6.1.3, iPhone5/iOS 6.1.4 and iPhone4/iOS7.0.

This commit removes the feature detection and introduces a userAgent string check to match iOS6+7 powered browsers.