Include header file
Move dt_from_struct_tm() and dt_to_struct_tm() to seperate source file
Moved core functions to separate source file
Minor refactor of days functions
API change

  dt_days_in_month()
    The function's behavior is now undefined if given month is out of range.

  dt_days_in_quarter()
    The function's behavior is now undefined if given quarter is out of range.