Feature request: output tzinfo compatible zone names

It's great that TZip allows you to assign the output of #find_by_zipcode to config.time_zone.

But for those of us working in Ruby only environments (non-rails) it would be nifty if TZip could produce names suitable for assigning to ENV[TZ].  For example, "Pacific Time (US & Canada)" would be "US/Pacific".  Etc.

This would allow something like:

```
ENV['TZ'] = TZip::tz_from_zipcode(90266)
```
