Add WKWebView for iOS8 devices

[nabilfreeman]

Apparently WKWebView has must better performance than UIWebView as it has the same power as native Safari...

[Haroenv]

Which in iOS 9 will be even a lot better (and actually basically Safari)

https://developer.apple.com/videos/wwdc/2015/?id=504

[alex3165]

I just PR the project for WKWebView support when iOS is up to 8.0, never done before but it seems works.

[nabilfreeman]

closed by #3!