#6 import from/export to iCal format

[Jay E]

Add ability to import events from an iCal format file, and the ability
to export events to an iCal format file. First iteration of import may
take shortcuts on importing repeating events, depending on how iCal
implements repeating events (e.g., repeating events with no end
date can't be represented in CGI Calendar). First iteration of export
may treat repeating events as all individual/separate events.