Fix for iOS device background images and small desktop windows

The CSS background-attachment: fixed; does not work properly on iOS devices. I wrote some code to remove that (from iOS). I also replaced the old way of checking for a tablet or device (by looking at the width of the window) with a check for the actual devices. This allows for small windows on desktop to still work normally. 

MORE IMPORTANTLY: The mobile devices are just a list I found online. Some may be missing. Just let me know if so.