Add support for time zones when calculating alarm trigger times

[cketti]

Unfortunately, this is a rather large pull request. However, the bulk of it is just moving things around, (hopefully) without changing the behavior. These commits are prefixed with [Refactor].

The functional changes are:

• Extracted the code to calculate the end date using start + duration to Task.end (for calendar events VEvent.getEndDate() was already doing all the work).
• Changed AlarmTriggerCalculator.alarmTriggerToMinutes() (previously ICalendar.vAlarmToMin()) to support time zones when calculating alarm trigger times
• A start time (or end time if the alarm is relative to the end) is now required to return the "alarm minutes". This simplifies the code, but required some changes to tests. (It's also in line with what RFC5545 requires.)