Support for exceptions

[bismark]

Would there be any interest in adding support for recurrence exceptions?

I've had to extend this lib to support exceptions for our needs so I wanted to open the discussion about pushing this functionality upstream.

In particular I've extended the NIF to support parsing EXDATE strings to return a list of date (times) and I've built a new Stream.resource that takes in the RRULE and a set of exception date(times) to generate the correct set of occurrences.