Linked timezones when generating data file

[luis-ca]

The timezones described in the olson files as linked to a principal timezone - such as Europe/Belgrade => Europe/Zagreb - are not being reflected in the generated files, and as such walltime throws an error when a zone like Euope/Zagreb is specified.

Not sure whether the data generation is incomplete or whether there is an alternative to this approach.

[jgable]

Linked timezones are not implemented right now. Mostly because we don't use them at Sprout. But, if you'd like to work out an implementation and make a PR we would welcome it.