Investigate performance drop since release of v2.0.1

[u01jmg3]

RE: #98

For some reason, I have a huge performance drop from commit dde72dc (~3s to load https://calendar.google.com/calendar/ical/efg.ludwigshafen%40gmail.com/public/basic.ics, parse it, and render all events in a month in a table) over f9f5866 (~15s) to 4de3963 (~45s).

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I have analysed all 20 commits since the release of v2.0.1 and found there to be 2 bottlenecks.

1. One surrounded using iCalDateToUnixTimestamp() in f9f5866 which always used a regex to parse the time zone
   • This has been changed in 9d44e5c to check that a time zone exists before trying to parse it
2. The second issue related to 48d285b and populating the DTSTART_array
   • The code was inefficient and has been improved in 9d44e5c

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• Finally it is worth noting that in b7b3ee6 the recurrence rule logic to handle EXDATE data when a count is imposed has been corrected so that EXDATEs don't count towards an imposed count.
  • This means there may be more event data resulting in increased processing times.