Add support for sensor calibration

[kylc]

Should be implemented as a req/resp message pair that will return offsets for each available sensor.

[kylc]

Two options I see right now:

1. Do it in the firmware. Send a request from the ground station to commence averaging on the quad.
2. Do it in the ground station. Send a request (maybe) for a series of sensor readings, do the averaging, and report it to the user. Maybe in the future send a message to set the offsets on the quad.

[kylc]

After talking to @yoos, we're probably going with option 2. Since calibration will require user input (turning the quad to various orientations, etc.) it makes sense to build it into the UI.