Add support for UTC times in iCal imports

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>

test/Makefile.am

@@ -48,6 +48,7 @@ TESTS = \
 	ical-004.sh \
 	ical-005.sh \
 	ical-006.sh \
+	ical-007.sh \
 	next-001.sh \
 	search-001.sh \
 	bug-002.sh \
@@ -111,4 +112,5 @@ EXTRA_DIST = \
 	data/ical-004.ical \
 	data/ical-005.ical \
 	data/ical-006.ical \
+	data/ical-007.ical \
 	data/todo