add methods (and approximations) for discrete integration of quaternions

[bluesquall]

Given an angular velocity and a timestep, produce a new quaternion.

This is one of the most common uses of quaternions, and it would be worth having functions to handle it in this utility library.

[bluesquall]

Implement these from my own notes, and/or refresh my memory on the mechanics.

see also:

• Applying angular velocity to quaternion
• Angular velocity from quaternions
• Efficient quaternion angular velocity
• Integrate angular velocity as a quaternion rotation
• Attitude from angular rate