Using RRULE[WKST] and a default first day of week

[jeromecombes]

I use French calendars and weeks start on Mondays, not on Sundays.
If we don't set the correct first day of week, we can have some bad results with recurrences.
I offer to update the class so that we can provide a default first day of week.
The class might also adapt the result depending on the RRULE[WKST] parameter.

From http://www.kanzaki.com/docs/ical/rrule.html :
An example where the days generated makes a difference because of

WKST:
  DTSTART;TZID=US-Eastern:19970805T090000
  RRULE:FREQ=WEEKLY;INTERVAL=2;COUNT=4;BYDAY=TU,SU;WKST=MO
  ==> (1997 EDT)Aug 5,10,19,24

changing only WKST from MO to SU, yields different results...

DTSTART;TZID=US-Eastern:19970805T090000
  RRULE:FREQ=WEEKLY;INTERVAL=2;COUNT=4;BYDAY=TU,SU;WKST=SU
  ==> (1997 EDT)August 5,17,19,31

My ICS example (the only difference between the 2 events is the WKST parameter) :

BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20160705T090000
DTEND;TZID=Europe/Paris:20160705T100000
RRULE:FREQ=WEEKLY;INTERVAL=2;COUNT=4;BYDAY=TU,SU;WKST=MO
DTSTAMP:20160514T170815Z
UID:tl6n0s54hq71d0hg3fm4zzzzt0@google.com
CREATED:20150624T141227Z
DESCRIPTION:
LAST-MODIFIED:20160707T075813Z
LOCATION:
SEQUENCE:1
STATUS:CONFIRMED
SUMMARY:TEST WKST MO
TRANSP:OPAQUE
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20160705T090000
DTEND;TZID=Europe/Paris:20160705T100000
RRULE:FREQ=WEEKLY;INTERVAL=2;COUNT=4;BYDAY=TU,SU;WKST=SU
DTSTAMP:20160514T170815Z
UID:tl6n0s54hq71d0hg3fmssszzt0@google.com
CREATED:20150624T141227Z
DESCRIPTION:
LAST-MODIFIED:20160707T075813Z
LOCATION:
SEQUENCE:1
STATUS:CONFIRMED
SUMMARY:TEST WKST SU
TRANSP:OPAQUE
END:VEVENT

[u01jmg3]

Fixed by PR #70