Refactor and add callbacks to navigation functions

[garetht]

This pull request

• refactors the navigational functions to use a builder function
• implements callbacks for the takeoff, landing, and navigational functions that are triggered upon the completion of those actions

The callbacks allow code similar to this to be written without the need for manual timeouts (using Q.js, and where q is a function that converts a callback-based function into a promise-based one).

Q.spawn(function*() {
  yield q(drone.connect);
  yield q(drone.setup);
  drone.calibrate();
  drone.startPing();
  yield q(drone.takeoff);
  yield Q.delay(1000);
  yield q(drone.up);
  yield q(drone.down);
  yield q(drone.land);
});

[voodootikigod]

Love it! I am driving up I-95 and will merge when I am home.