Don't try to sleep for a negative number of seconds
Use the thumbnail for the mean brightness check, because it's faster than using the full image
Replace CONFIGS with a full range of 1/3 stop increments

Note that the Raspberry Pi camera module only supports exposures up to 6
seconds at the long end (I'm not sure about the short end, but the shutter time
is specified in microseconds and 1/8000 is 1.25x10^-4 so it's worth a go and
will be better than 1/1600).

Now we have quite fine control, the threshold for 'ideal brightness' is reduced
to 25000 +/- 10%

Initial exposure is 1/80th at ISO 100
Improved presentation and reliabilty of start/stop in README.md