New API (2.0.0-dev)

* 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``).
* Added ``RRule.parseString``
* Added ``RRule.parseText``
* Added ``RRule.optionsToString``