Update zone.pp

please consider merging these two changes:
the first ($purge_zones) allows one to override the full purge of any firewall zone that's unknown to this module,
the second (sources example) illustrates the format one should use for mapping IP prefixes onto zones. We find this feature quite useful. :)
Thanks!
Merge pull request #3 from RamsesRodenburg/patch-2

Update zone.pp