Fix VTIMEZONE DTSTART format and TZOFFSETFROM calculation

[icvalyssa]

Fix for Issue #53

• Remove inline TZID from VTIMEZONE -> DTSTART (regardless of dateTimeFormat) for compatibility with Google and adherence to https://tools.ietf.org/html/rfc5545#page-66. The inline TZID for the VEVENT DTSTART/DTEND remains intact and dependent on dateTimeFormat.
  Old: DTSTART;TZID=+00:00:20201101T090000
  Fixed: DTSTART:20201101T090000
• Set VTIMEZONE -> DTSTART to local time as stated in https://tools.ietf.org/html/rfc5545#page-66 ("DTSTART in this usage must be specified as a date with a local time value"). Since getTransitions() returns UTC times, use the previous offset to calculate local time.
  Old: 20201101T090000 (Los Angeles 2am transition time, UTC)
  Fixed: 20201101T020000 (Los Angeles 2am transition time, local)
• Update the VTIMEZONE -> TZOFFSETFROM calculation to look back more than 1 hour to get the previous timezone offset. In some cases, the previous offset was not different from the new offset, possibly due to the 100-second look back putting it in the same offset due to the transition.

Old:

BEGIN:VTIMEZONE
TZID:Australia/Sydney
BEGIN:STANDARD
DTSTART:20200405T020000
TZOFFSETTO:+1000
TZOFFSETFROM:+1000

Fixed:

BEGIN:VTIMEZONE
TZID:Australia/Sydney
BEGIN:STANDARD
DTSTART:20200405T030000
TZOFFSETTO:+1000
TZOFFSETFROM:+1100

[jasvrcek]

@icvalyssa this is great, thank you! Can you please update the .ics example files in /tests to reflect your format change, so the tests can pass? See https://travis-ci.org/github/jasvrcek/ICS/jobs/734157770.

[dereuromark]

Is this fix still relevant?