RRULE string parsing

[Zacqary]

Parsing of RRULE strings is part of the iCalendar RFC. We didn't need this in the original Kibana library, but the open source community would likely benefit from a string parsing method.

Acceptance Criteria

• Implement an RRule.fromString() method

[pippinmole]

This is required for the package if its to be taken seriously :)