Just refactor some awkward code
Uniquify events as they're imported

Note that this doesn't uniquify them against existing events, so the
existing duplicate_of logic should still exist.

Fixes https://code.google.com/p/calagator/issues/detail?id=241
Remove a declared-but-unused variable
Add support for TZID=GMT

RiCal mostly supports TZID tags, but doesn't recognize GMT. We'll just
assume GMT means UTC (which may elide details of British DST).
Implement a skipped test
Turn a skipped test into an issue [#146]

https://github.com/calagator/calagator/issues/146
Merge pull request #147 from AndrewLorente/alorente/unskip-tests

Tidy up skipped tests