Allow parsing of jcal files

[niccokunzmann]

icalendar adds jcal support.

It would be nice to support jcal in a way that makes it really easy to parse.

The proposed interface is this:

• merge a icalendar.Calendar list
• merge a list of strings or bytes containing iCalendar data (json/ics)

merging jCal and iCal should be detected automatically and there should not be anything users have to do to make it work.
The output should be iCal