Add support for caldav, icloud, ics calendars

[anthonyscorrea]

Addresses issue #42

[mendhak]

Hey thanks for this PR some good features in here. Since I'll need to support this going forward, I've done some testing and reviewing to see how to incorporate it. A few main points I want to mention:

I tried signing up for ICloud but I was unable to create an account without providing a phone number (and credit card). That's too invasive for me and I'm not willing to go through with it or fake it. For that reason I can't take the ICloud feature.

I can test CalDav using NextCloud, that should be good enough.

I wasn't able to get the ICS feature working without making the datetimes timezone aware, I kept getting errors about comparing datetime naive and datetime aware. After that I noticed that the ICS feature isn't showing recurring events. I had a look at the icalendar page and it points at a few other projects to try. icalevents seems to cover the right set of features.

I appreciate you may not have time to make those changes, so I could take it over. It could also give me a chance to refactor these features into a proper calendar provider for better maintenance.

[mendhak]

Today I spent time testing caldav and also tested with recurring events in caldav.
I think recurring events aren't possible yet, it's waiting on a pull request here: python-caldav/caldav#157
Or it's possible but I couldn't figure out how.

[mendhak]

OK I figured out recurring events with caldav, this was harder than necessary, it's a terrifying 'standard'.

Also I've encountered different behaviors across different caldav providers like NextCloud and FastMail. I'm adding some code to deal with these discrepancies.

[mendhak]

All calendar types now moved into a provider. README instructions updated.
I've also cleaned up the instructions a bit, and moved the pip install packages into a requirements.txt
Not merging just yet, still testing.