NightlyBase doesn't convert string in dayOfWeek param

No conversion is made of the value in dayOfWeek if it's a string. The
value is directly passed to croniter without proper conversion.
This fix handles both single string values and multiple comma separated
values as well as abbreviation in the string. ("0" "5,6" "0,fri" et.c.)
Is the comment in the latest commit enough or should I mention that croniter handles any invalid values?