lots of microcontroller/sensor/grapevine changes

added signal handler for SIGINT signal so microcontroller modules exit cleanly

added functionality for sensor faker to send changing values, support for the start switch and the second mag/accel, ability to modify from the cmd line how often sensor data is sent

fixed bug in sensor class so that it properly calls the callback function

re-tabbed the uc sensor grapevine interface so it uses spaces instead of tabs

tweaked robosub_settings file, changed names for sensors to reflect the possibility of more than one type of each sensor, also renamed the compass to magnetometer

added grapevine debugging module that prints the raw sensor data received over the grapevine