I've been developing the PID code for about a week now, and I finally got the PID code to work! It works by implementing two PID loops, one for the roll axis, and another for the Pitch axis. The PID loop's output is then converted and output as servo motion, which controls the flaps of the glider. 

Possible Improvements: I'm thinking that using 1 3d quaternion and 1 PID loop would be a lot better, as the flaps can move more efficiently and accurately with less errors. 

Video: