More clever handling of images

- Instead of reading all the images three times, we can actually use
  list comprehension on the "Images" to calculate Max, Mean and STD...
- Appending the images to an empty image makes it easy to calculate the
  sum of all projections and the mean of the dark images for display at
  the end.
- Writing to the log file during the decision loop should speed up the
  script a tiny bit
- Save output images after we're showing everything to the user should
  make the script to seem to be faster :)
Final result (Brightness per Exposuretime)

- Another `pep8` run
- Formatting log file and plot title output nicely
- At the end, display image row sorted with the CMOS exposure time. This
  makes it possible to chooser the 'best' exposure time, i.e. the one
  where we loose the leas photons due to the readout time.