Add CalDAV calendar support for multiple providers

[bobuk]

@L0ndra check out the first (and dirty approach) for caldav support. It is NOT well tested and should be treated as a dirty initial thoughts about not-only-google refactoring

[L0ndra]

What do you think about moving Calendar creation to separate module, so you shouldn't know anything about provider inside the commands logic?

[L0ndra]

i am thinking about something like getAllCals that return array of CalendarProvider

[L0ndra]

All other aspects appear satisfactory.

[bobuk]

Not quiet sure about quality but does it look like you're talking about?

[L0ndra]

yes the idea is correct, i left a comment about calendar provider, it may simplify the interface