Implement ElevatorAttendant

[mcolinj]

We need a command like controller to move the elevator. The elevator position has 6 set points (determined by 6 buttons). Instead of the gyro, as was use to build the Turner object, we will read an encoder. I think the physics model has the idea of an encoder for the motors (driving forward and backward). So you could write and debug the elevator controls by pretending that the robot is the elevator and just program it to drive forward (up) or backward (down) to preset positions on the floor (determined by the encoder). The code should look pretty much like the stuff done for the Turner (in the PID-v2 branch), except the ElevatorAttendant will provide data for the forward/backward motion of the drivetrain instead of the turning.

[mcolinj]

This is mostly implemented now. See issues #31 and #34 for more things remaining to do.