Fix opaque navbar

The math behind calculating how much of the keyboard was covering the UIScrollView was being done in different coordinates, which was working when the UINavigationBar is translucent because the UIViewController's view covers the whole UIWindow, but not when that is opaque. 

Also updated the math calculating the UIKeyboard's Y position while being dragged.

Fixes bug #405
let's rework this: 

````objective-c
CGFloat contextViewOriginY = CGRectGetMinY(self.contextView.frame);
````