Remove & replace iCal support with dependency on icalendar gem

[avit]

Our implementation of iCal parsing is untested and lacking features. There exists a solid maintained icalendar gem that could support conversion to/from .ics format for us.

IceCube is useful for generating the occurrence times from defined schedules, so these two gems could work well together and allow each to focus on its strengths.

This dependency could be optional.

If anyone has experience with the icalendar gem, I'd like to hear your thoughts on this proposal.

[seejohnrun]

Big fan of this proposal - much more flexible and less error-prone since this isn't a big part of IceCube's functionality

[espen]

The parsing I supply to the icalendar gem is very basic but never had any major issues with parsing or generating ics with it. I have however experienced some timezone problems but I don't recall if that was just some tests or some few errors in production. But given icalendar/icalendar#167 it doesn't seem like the parsing is that solid at the moment.

Also note that https://github.com/icalendar/icalendar-recurrence actually uses IceCube for icalendar parsing.