Implement recurring events and todos

[jackkamm]

Fixes #150

This seems to be working, but I plan to add some unit tests before merging.

Also should update the documentation to note this is supported now, but only for simple cases (no iCal UNTIL or COUNT properties), and also to note some subtleties in the case of TODOs (e.g. the user may want to set org-icalendar-todo-unscheduled-start).

I'm opening the PR now for visibility and so people can test it out if they like.