Fix pitch calibration and added pitch guidelines

Fixed the pitch calibration. Previously, the artwork was inconsistent, leading to a larger gap between 10 and 20 degrees than 0 and 10 degrees.

Previously, at 20 degrees the 20 degree mark and the level line would not have touched. Now they do, as can be seen in the screenshot
![Screen Shot 2013-04-21 at 4 47 12 PM](https://f.cloud.github.com/assets/1118185/406449/4f25e1c8-aa93-11e2-9091-ec9e0914c93d.png)

I also added a pitch desired indicator:
![Screen Shot 2013-04-21 at 4 46 39 PM](https://f.cloud.github.com/assets/1118185/406448/4f2493fe-aa93-11e2-94f6-71960d35d789.png)

Edit: I also added some fixes to angle wrapping.