a great refactor in DayCount

DayCount now has two new methods: daysinunit and unitsize. I am not
100% sure about that refactor, maybe I should have a Unit class and
this class would have an instance of DayCount, but for now that's the
way it goes.
test adapted to the new DayCount powers
added new test for DayCount
clean up
GenericPeriod class created

Now both FixedTimePeriod and DateRangePeriod inherit methods from
GenericPeriod
numberof() method have been replaced by size()