Json output

Add support for basic JSON generation in Calendars
unrelated note, I would probably wrap configuration into new calendar, so I could just call
```ruby
 cal = SimpleIcloudCalendar::Calendar.new(
   ical_url: ical_url, 
   start_date: start_date, 
   end_date: end_date)
```
and have the `Calendar` entry be clever enough to realize it's getting arguments to a `config`, not a `config` object.