Add gem signing options if a key file is found.

Don't rely on the key being present in ~/.gem to auto-sign the gem.
Fix typo.
Support loading system zoneinfo data on FreeBSD, OpenBSD and Solaris.

- Support locating the iso3166.tab file when it is not included in the
  zoneinfo directory.
- Handle the four column iso3166 tab file format used by FreeBSD.
- Support finding the iso3166.tab and zone.tab files using alternative
  names on Solaris (tab/country.tab and tab/zone_sun.tab).
- Support loading country zone data from the non-standard five column
  zone_sun.tab format.

Resolves #15.