Activates subtraction operator with right side tod object

[hirokishirai]

Regarding addition and subtraction operators, I wanted functions like following.

right_side = Tod::TimeOfDay.new(10,0,0)
# => #<Tod::TimeOfDay:0x007fe98a5a8c70 @hour=10, @minute=0, @second=0, @second_of_day=36000>
left_side = Tod::TimeOfDay.new(12,0,30)
# => #<Tod::TimeOfDay:0x007fe98a55b178 @hour=12, @minute=0, @second=30, @second_of_day=43230>
result = right_side - left_side
# => #<Tod::TimeOfDay:0x007fe98a4db1a8 @hour=21, @minute=59, @second=30, @second_of_day=79170>

This PR enables addition and subtraction when Tod::TimeOfDay Object is on the operators right side.

[jackc]

I'm not convinced that adding times makes sense. What does it mean to add 3AM to 8AM?

I think we should follow the precedent of the Ruby standard Time object. Subtracting times is okay, adding them is not.

Limit this PR to subtraction and I'll merge it.

[hirokishirai]

I see. I understand.
I'll remove the changed codes of addition.

[jackc]

Thanks!