BREAKING: fix VEvent ical spec errors

• fixed VEvent to allow multiple rrules / exrules as per the ical spec.
  • This also included appropriate changes in ical-tools serializeToICal() and parseICal() functions.
• updated Dates to not change the timezone associated with Dates#adapters. This means that a date in Dates#adapters may not have the same timezone as Dates#timezone (dates yielded by Dates are still updated to have the same timezone as Dates#timezone, however). This distinction can be important when serializing a Dates object as it ensures the original timezones associated with the underlying dates are preserved.
• added ability to set Rule#options via Rule#set().
• added ability to pass whole Rule and Dates objects to the VEvent constructor.
• added VEvent#set(), VEvent#add(), and VEvent#remove().