API improvements

• It's now possible to put things into calendars without first
  creating valid ical data (just pass things like tdstart etc to the
  save_event/save_journal/save_todo methods).

• The DAVClient object can now work as a context manager

Major bugfixes

The authentication code was "fixed" in 0.8.x to work around some weird
bug in one specific calendar server, but this introduced lots of
problems for other servers. It was then rewritten in 0.9, but still
causing (regression) problems with some servers deviating slightly
from the relevant RFCs. With some help from Bjoern Kahl, Markus
Behrens and Michael Thingnes I believe it should now be quite robust.
(IMO, this work does not belong in the caldav library, ideally it
should suffice to pass username and password to the requests library).

Other things

Some other minor bugfixes and more work on the test framework