Added Timezone support

Added basic timezone support (TZID property) for the entire calendar based on 

http://www.kanzaki.com/docs/ical/tzid.html
and 
http://stackoverflow.com/questions/7626114/ics-timezone-not-working

I didn't add [VTIMEZONE](http://www.kanzaki.com/docs/ical/vtimezone.html
) support since many Calendar apps (Google Calendar and iCal) have local TZDB fallbacks and hence work fine as long as the added timezone is in [this list](http://en.wikipedia.org/wiki/Tz_database).