I spent quite some time to learn a bit (very few in fact) of python as the library picamera gives acces to a lot of setting for the camera module. I made as well a small circuit around the MCP3426 ADC using some photo-resistors.
Right now I've only define some thresholds to change the exposure time. Which explains why there is some brightness jumps in the video below. However it is a quite promizing solution, and I'll continue to try to improve the exposure compensation.
These python tests are on GitHub and needs the picamera, PIL and numpy libraries.