Updates postgis to 1.5.8 for Postgres 9.1, enforces installation of known good version of sci-libs/geos, eliminates repeat installation of postgis packages
Updates Timezone README to address database considerations.
Merge pull request #1 from tpoland/DATA-334

Updates Timezone README to address database considerations.
Merge pull request #178 from tpoland/master

Updates postgis packages for PostgreSQL 9.1