output of subtraction and addition

I was surprised to find that a duration is represented with a `Tod::TimeOfDay`. This seems semantically incorrect. It seems it should be either some sort of new class representing a period of time or just a float representing seconds, as is done in other libraries.

Would you be open to changing this behavior for the next major version release?

```
> noon = Tod::TimeOfDay.new(12)
> afternoon = Tod::TimeOfDay.new(14)
> d = afternoon-noon
=> #
> d.to_i / 60 / 60
=> 2
```

Start by reproducing the subtraction example with Tod::TimeOfDay and inspect how addition, subtraction, and to_i represent durations. The issue leaves the replacement type undecided, so research the existing behavior and comparable Ruby libraries before proposing a design for the next major release.

Written by the indexing model from the issue text.