WorkdaysBetween: Use default zci is_cached

zci is_cached => 1 is now the default, so there is no reason to specify
it explicitly.
WeekdaysBetween: Don't use integer pragma

Instead of using the integer pragma for the whole file, simply cast the
$num_weeks calculation to an integer before assignment.
WorkdaysBetween: Match on "business days between"
WorkdaysBetween: Parse dates in another subroutine
WorkdaysBetween: Allow DD/MM/YYYY date format
WorkdaysBetween: Add ridiculous date format tests
WorkdaysBetween: Add comments to clarify remainder calculations
WorkdaysBetween: Allow date format MMM DD YYYY
WorkdaysBetween: Allow comma in MMM DD, YYYY format
WorkdaysBetween: Allow date format M/D/YYYY