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/3659090/1435409/ad8c2286-413d-11e3-9d94-f6dda931aec0.jpg)