Review hcalendar support

[da4089]

Things to consider:

• h-event (http://microformats.org/wiki/h-event) seems to be a newer version of hcalendar.
  • Should we change?
  • Support both separately?
  • Support both concurrently, applying both markups within the same generated HTML?
• There's no documentation I could find on using hcalendar
  • I discovered that it can be used as the behavior argument for serialize(), only because that ability was added in the same commit.
• HCalendar is added to the behaviour registry, but to my knowledge, that's used solely for looking up behaviours when parsing, and we don't parse hcalendar format inputs
  • I don't think we should be adding a hcalendar parser?
    • If we do, see https://github.com/microformats/mf2py
• Unit test coverage
• vCard support
  • There's both hcard (v1) and h-card (v2) defined