Support quoted timezones in iCal feeds

[stephenharris]

The iCal specification seems to suggest that timezone identifiers (TZID) should not contain double quotes or colons (SAFE-CHAR).

However, this timezone popped up in an iCal feed:

 DTSTART;TZID="(GMT +01:00)":20140712T100000

This caused a problem because the parser simply splits the string at the first colon.

In so much as it doesn't alter existing behaviour for correctly formatted iCal feeds, Event Organiser should instead ignore any colons inside quotation marks.