Support RRULE export in ICS from libphutil

Summary: Ref T10747. This formats RRULEs for export into ICS files.

Test Plan:

• Imported event into Calendar.app. Events appeared properly, except that the extra event in 2009 did not show up (apparently, Calendar.app does not support RDATE?).
• Imported event into Google Calendar. Events appeared properly (including RDATE and EXDATE).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16648