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/ical-007.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+
+. "${TEST_INIT:-./test-init.sh}"
+
+if [ "$1" = 'actual' ]; then
+  mkdir .calcurse || exit 1
+  cp "$DATA_DIR/conf" .calcurse || exit 1
+  TZ="America/New_York" "$CALCURSE" -D "$PWD/.calcurse" \
+    -i "$DATA_DIR/ical-007.ical"
+  "$CALCURSE" -D "$PWD/.calcurse" -s2015-02-23
+  rm -rf .calcurse || exit 1
+elif [ "$1" = 'expected' ]; then
+  cat <<EOD
+Import process report: 0018 lines read
+2 apps / 0 events / 0 todos / 0 skipped
+02/23/15:
+ - 06:00 -> 07:00
+	UTC
+ - 11:00 -> 12:00
+	Local time
+EOD
+else
+  ./run-test "$0"
+fi