Fixing requestAnimationFrame & cancelAnimationFrame

With the previous implementation, Chrome complains:
    "'webkitCancelAnimationFrame' is vendor-specific. Please use the standard 'cancelAnimationFrame' instead."
This fixes that by reusing (copy-paste) the code from angular.js
A better way would be to use ionic.requestAnimationFrame and ionic.cancelAnimationFrame everywhere, but that is more involved and risky...