Copter: Add support for V-Tail Quads

I feel like this is really close. I need to do some PID tuning to see if this motor mixing works well, but It'll at least give people somewhere to start.

Something to note is that the pitch is backwards (not sure how to change my values at the moment to fix that) - so I just reversed pitch on my remote, and it works fine.

For motor rotation and position, it's the exact same as the Quad X.