Add support of multiple RRULE

[fabien-michel]

When an event got multiple RRULE.
Exemple:

RRULE:FREQ=WEEKLY;BYDAY=TH;COUNT=20
RRULE:FREQ=MONTHLY;BYDAY=2MO;COUNT=2

It was surprisingly easy to do, and surprising that it hadn't been done before. So I doubt I'm missing anything.

[niccokunzmann]

Uh nice!

Would you like to add a changelog entry, too?
Tell me when you are ready and I review it and merge. Looks good!

[fabien-michel]

Ok, I think you can review it.
Be really careful, all the tests are passing but I'm not really confident about changes I made. Especially the part modifying rdate conditions where I changed single-rrule UNTIL value to the max UNTIL value.

[niccokunzmann]

Thanks, this looks really good!

I am pretty sure that this is better than before. Since this is a test-driven library, I am confident that you did not break anything! Thanks for testing, too, because that allows us to be confident!

[niccokunzmann]

@fabien-michel Thanks a lot for the nice contribution. Feel free to add a new PR if you find that you would like to change the code a bit more!

[niccokunzmann]

I release this under v2.2.1.

[niccokunzmann]

Because of the clean work, I give you write access. With this, you can also start to release a new version, review and merge other PRs.