Trim leveling: Use transmitter trim to refine level

This is useful after trimming an aircraft for level flight in
attitude mode to set the roll and pitch that should be considered
level.

I tested it by introducing 10 degrees of error in the roll, flying and trimming it out, and then using the trim button.  Works great.

![Screen Shot 2013-03-15 at 9 09 55 AM](https://f.cloud.github.com/assets/618346/263743/0b1df88e-8d7a-11e2-8851-1ed37b92af64.png)