Share specs for Time, Date and DateTime #strftime since they have much in common

* Improved a couple specs along the way
* Ensure there are specs for every documented conversion specifier

I had a couple questions:
* Is the location for the new files fine?
* What would be a good naming for "specs about the date (year,month,day)" and "specs about the time" ?
* Is it OK to use `before :all` this way to pass data (the constructors) ?