refactoring and enhanced support for the OLED

this is a rebase of my "enhancements" branch against current master. it adds a bunch of hidraw commands for manipulating the OLED, like drawing bitmap graphics or a larger font. it also uses the PWM timer to track wall time, for some power savings and more tunable blinking. the new commands are described in the README.md.

in the process, i split the main file out into a few modules (display, controller, etc) to make it a little easier to apply future patches.

you're welcome to squash this into a single commit if you accept it -- the individual commits are just checkpoints.