Setpoint chevrons

When the altitude or airspeed bugs went off the ticker, there was no indication of which way the error was. This PR adds chevrons to the PFD, providing some indication of setpoint datan when the setpoints are off scale.

Normal airspeed and altitude bugs.
![Screen Shot 2013-04-19 at 2 27 33 PM](https://f.cloud.github.com/assets/1118185/401872/ba2786a0-a8ec-11e2-8a7b-3c110a946417.png)

When the bugs go off-ticker-tape, they are replaced by chevrons.
![Screen Shot 2013-04-19 at 2 27 20 PM](https://f.cloud.github.com/assets/1118185/401873/bcf1233c-a8ec-11e2-908e-a63cf4904e02.png)

P.S. In my opinion, the chevrons are a bit too narrow and tall. They'd probably look better squatter.