v2.0.0

• Fixed a February 28-related issue.
• More flexible, backwards-incompatible API:
  • freq is now options.freq.
  • options.cache is now noCache.
  • iterator has to return true
  • dtstart and options arguments removed from RRule.fromString
    (use RRule.parseString and modify options manually instead).
  • today argument removed from Rule.prototype.toText
    (never actually used).
  • rule.toString() now includes DTSTART (if explicitely specified
    in options).
  • Day constants .clone is now .nth, eg. RRule.FR.nth(-1)
    (last Friday).
• Added RRule.parseString
• Added RRule.parseText
• Added RRule.optionsToString