Separate API and Program

[matt-garman]

This is a fairly dramatic change, as I also reformatted most of the code to be a bit more consistent.

In short, I separated the details of the USB functionality from the actual CLI program. I also hid all the details of the USB code into a simple API:

typedef struct usb_temper* usb_temper_t;

usb_temper_t usb_temper_init(int devicenum, int debug, int calibration);
float        usb_temper_get_tempc(usb_temper_t usb_temper);
int          usb_temper_finish(usb_temper_t* usb_temper);

The goal here is just to make everything more modular. This also sets the stage for my next PR, which adds SQLite logging capabilities...