Add timezone parsing

[alkim0]

Fixes SimpleMobileTools/Simple-Calendar#1249. It will also fix #71.

Two things to note:

1. This doesn't parse BEGIN:VTIMEZONE and assumes the timezone is one of the known timezones (e.g., America/New_York). This is just to get things working and is what etar does right now.
2. The imported event's timezone is not preserved; it's converted to the local timezone. I didn't want to think about what to do when DTSTART and DTEND have different timezones (technically possible), since the app only stores one timezone per event.

[naveensingh]

Seems to work just fine, thanks for the fix :)