Replace custom fact with executable fact

Should solve #4

facts.d/sabayon.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+if [ -f /etc/sabayon-release ]; then
+        echo "operatingsystem=Sabayon"
+fi

lib/facter/operatingsystem.rb

@@ -1,11 +0,0 @@
-Facter.add(:operatingsystem) do
-   # Sabayon Linux is a variant of Gentoo so this resolution needs to come
-   # before the Gentoo resolution.
-   has_weight(100)
-   confine kernel: :linux, lsbdistid: :sabayon
-
-   setcode do
-     'Sabayon'
-   end
- end
-