Adds compatibility for time_select helper and fixes "overlaps?" for certain cases

[kennyeni]

Hi,

This fixes might be specific for my project, but I'm sharing them in case they are helpful for the community.

1. time_select compatibility: rails has a helper method that renders a time selector on views, however, the object must have Date and Time attributes (although it does ignore the Date attributes...), so this little change adds a placeholder day that will be ignored AFAIK.
2. overlaps?: as I understood this method, it should return a boolean that represents if two "Shifts" overlap in time, I found some cases that failed according to my logic, for example, calling

shift1 = Tod::Shift.new(Tod::TimeOfDay.new(7), Tod::TimeOfDay.new(2))
shift2 = Tod::Shift.new(Tod::TimeOfDay.new(1), Tod::TimeOfDay.new(4))

would represent something like this:

And overlaps? should return true AFAIK, which it does not.

I added a few more test cases on tests, as there are some edge cases to be considered.

One more time, I don't know if this is the expected behavior for all projects, and how the author planned them, but this proved to be useful in my particular case

[jackc]

I merged in the overlaps changes, thanks!

I didn't merge in the time_select compatibility. I think you can use the time_select option :ignore_date instead.