According to the iCal standard (4.3.12 Time):
UTC time, or absolute time, is identified by a LATIN CAPITAL LETTER
Z suffix character (US-ASCII decimal 90), the UTC designator,
appended to the time value.
Parse such time values properly when importing iCal files.
Fixes GitHub issue #3.
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>
