Validate timezones

[garrett]

Talk datetimes (start and stop) are now parsed in the validate.rb script. However, incorrect timezones passed to Ruby's DateTime.parse are ignored.

This should be checked so that:

1. The timezone actually does exist
2. The daylight savings time matches the date

Obviously, we'd want to rely on existing Ruby library to check this. Hopefully standard Ruby can handle it. Otherwise, we might need to pull in an additional gem like chronic.

[duck-rh]

very good idea (after the recent build failure I saw on IRC).

Time and DateTime all fail to report invalid TZ, and the doc says it does not care about DST.
chronic is no more maintained (latest version in 2013); nevertheless it could have done the job, but it fails to report invalid TZ. Using strptime() with a %Z pattern does not work either. Rails extended classes do not seem to help much either. Honestly I wonder how to do that.