Actually care about --days and --events

Fix #107
Test for DbSync
refactoring: dbtool now only operates on one calendar

also started renaming `account` -> `calendar` (account was a leftover
from pycarddav)
refactoring
more refactoring
unused code elimination
get_vevent_from_db() -> get()
typo
bugfix
events deleted from the vdir should now be deleted from db as well

this should fix #106 on github
khalendar.Calendar docstring
unused code elimination
more refactoring in backend
increased coverage for Event() & some small fixes

* FIX: in some cases non-ascii characters were printed even if
  unicode_symbols is set to False in the config

* FIX: events with different start and end timezones are now properly
  exported (the end timezone was disregarded when building an icalendar,
  but since timezones cannot be edited anyway, this shouldn't have
  caused any problems)
release v0.3.1
set version to 0.4-dev
doc version dynamic
Show nicer error message before building docs

See #108
Port CLI to click
Add click to deps