Now that we have three gyros and working sampling code with feedback
to the EFIS, it's time to start on the autopilot code.  I've drawn
up some thoughts on the subject and am looking for feedback.

The control loop is run on a 60 Hz cycle, which is roughly one full
servo PWM frame.  We can't make changes any faster to the servos,
so this seems about right.  Meanwhile, sensor sampling is happening
in realtime and being averaged (weighted average) between control
loop cycles.

 - Maintain attitude:
   - Level?     Do nothing
   - Steady?    Slow, proportional response.
   - Righting? 	Reduce the response
   - Tipping?   Fast, proportional response.
 - Parse serial input
 - Do status output

Interrupts:
 - A->D (gyros, fuel, etc)
 - Pulse counting
 - UART I/O
 - Servo PWM output

The new features that will have to be added before tomorrow's test flight:

 - Remote override from the serial port.  We will be giving the realtime
   system control of the cyclic servos and need to make sure that there
   is a way to recover control to the groundstation should things go off.

 - Remote attitude zeroing.  Right now it is a physical button on the
   STK200 board.  I don't want to have to get too close to hit it.

 - Slow servo response.  Rather than zipping to the new position,
   it will slowly move to there.

 - Other stuff to make it work.
