Added 8 rotor Coaxial Rotor mixer

Mixer based on the X8 APM setup: http://copter.ardupilot.com/wp-content/uploads/sites/2/2013/04/FRAMES_X8.jpg
![frames_x8](https://f.cloud.github.com/assets/3659090/1444691/af1f9746-420e-11e3-9080-e95b9dded608.jpg)