[jcxue/quat2angle] add utility function to convert quaternion to angles

[jcxue]

add a function to convert quaternion to Euler angles.

[pwaller]

I tried extending the test in the obvious way (testing XYX, etc) but I can't make sense of the results and I've run out of time for now. In that case X and Z end up flipped on the output and I'm not sure whether this makes sense or not.

This might be an interesting function to test by round-tripping lots of random numbers through it.