Adjustable group size

The first commit makes the group (bin) size adjustable on-the-fly via two keyboard shortcuts ("+" and "-") making it possible to "zoom out" the orderbook while goxtool is running. I've set the step size to 1, but feel free to change it if you don't think it's appropriate. Perhaps this should be made a config option as well?

The other commit changes the main loop so it short-circuits once a key is detected since the keys are mutually exclusive so there is no need to check all the ifs.