Use drawViewHierarchyInRect for iOS > 7 for rendering UIImage from an UIView

On iOS 7 we could use `drawViewHierarchyInRect:afterScreenUpdates:` for rendering the current `UIView`.

With this change the rendered `UIImage` is exactly that what it's shown on device. 
With the current implementation this is not so. For example `UIPickerView` is not correctly displayed and completely broken with iOS 7.1.

I implemented it with backward compatibility for iOS 6.x.