Add Lua function to make sensor reading explicit.

- Allows us to precisely control the relative timing between
  the sensor reading and the PID control.
- Added methods to "Quadcopter" to return the latest accel, gyro,
  and GPS readings.
- The accel and gyro tubes seem to take a few ticks to warm up
  before we can read data from them.  This might mean that there
  is a lag on these sensor readings.