Fixing two issues

Fixes #148 iOS address bar and backstrech image height
Fixes #218 Backstretch resize() method doesn't anticipate on unloaded image (ratio)

The fix for #148 is most interesting I think:

In the case of an iPhone, it will only trigger the first time and on each orientationchange. Because the orientationchange-event is not reliable, I just check if the rootWidth has changed.

Then if it does continue to process, it then checks if makes sure the rootHeight is always calculated without the address bar. At first load iOS always reports the full height, but on orientationchange it doesn't.

This way the image does not resize after scrolling the address bar out of view. Of course this means the bottom part of the image is not visible when the address bar is shown, but that's something we can all live with I guess :)