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