More improvements to duration handling.
Added some division operations. Can now divide Durations by Durations or by ints.
The rules:
1. Duration/Duration => int (a proportion: 9 minutes/3 minutes = 3)
2. Duration/int => Duration (a division: 9 minutes/3 = 3 minutes)
3. Duration%Duration => Duration (a remainder: 9 minutes/7 minutes leaves 2 minutes remaining)
4. Duration%int => Duration (also a remainder: 9 minutes/7 leaves 2 minutes remaining)