Recognize Gentoo in Facter 1.7.0

Gentoo was added as its own osfamily in Facter 1.7.0 (see http://projects.puppetlabs.com/issues/17029).

This means Gentoo is not recognized by the params class.

Fixes GH-83.
update spec for Gentoo & Facter 1.7
update puppet-system tests for Gentoo & Facter 1.7
Merge pull request #104 from 3flex/gentoo-facter-1.7-support

Gentoo facter 1.7 support