Add new feature to time the stacking process and then display
the elapsed time at the end of the stack.
Refactored code for saving intermediate frames.

Code for saving intermediate frames has been moved from process_light_frame
into it's own function.  This is called from the main loop not
process_light_frame so extra paramaters can be removed meaning the process
function has a much clearer purpose and signature.
Add new feature live display update

Adds new optional feature where the user can have the image display and update
during the process of stacking.  Works with all other features but slows the
process dramatically as updating the display is expensive.
Bump version number
Update README.txt to include 1.6 version changes.