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) ?
* GNU modificators are only tested for a couple specifiers right now, as it was, MRI does test it for about every specifier, what would be a good approach? https://github.com/ruby/ruby/blob/trunk/test/date/test_date_strftime.rb