API change

Renamed following functions:

    dt_first_day_of_year    ->  dt_start_of_year
    dt_first_day_of_month   ->  dt_start_of_month
    dt_first_day_of_quarter ->  dt_start_of_quarter
    dt_first_day_of_week    ->  dt_start_of_week

    dt_last_day_of_year     ->  dt_end_of_year
    dt_last_day_of_month    ->  dt_end_of_month
    dt_last_day_of_quarter  ->  dt_end_of_quarter
    dt_last_day_of_week     ->  dt_end_of_week
Minor refactoring of easter calculation
Restructured Makefile