Apply firewall resources alphabetically

This will apply firewall resources by alphabetical order.
This greatly simplify Module usage IMHO.

Just:

```puppet
include ::firewall

resources { "firewall":
  purge => true
}

firewall { '000 accept all icmp':
  proto   => 'icmp',
  action  => 'accept',
}

firewall { '001 accept all to lo interface':
  proto   => 'all',
  iniface => 'lo',
  action  => 'accept',
}

firewall { '002 accept related established rules':
  proto   => 'all',
  ctstate => ['RELATED', 'ESTABLISHED'],
  action  => 'accept',
}

firewall { '999 drop all':
  proto   => 'all',
  action  => 'drop',
}
```

No more 
```puppet
Firewall {
  before  => Class['my_fw::post'],
  require => Class['my_fw::pre'],
}
class { ['my_fw::pre', 'my_fw::post']: }
```