Bugfixes, time zone handling and misc

v0.12 mostly contains bugfixes and refreshed documentation as compared to v0.11.

I've hopefully gotten timezone handling correct by now.

Since v0.12 is not bug-compatible with v0.11, it is v0.12 and not v0.11.1.

I should probably be better at doing patch releases in the future, but I need some CI-system that automatically can run tests towards different caldav servers and different python versions before I can do rapid patch-level releases.