Update geoip.rb for usage with Hash-Fields

If your ip-field is in a hash-field you can specify the index of the hash.
E.g.
== Json Input ==
```json
{ "@timestamp": "2014-04-16T04:42:20+0200", "client_data": {"name": "Max", "ip": "127.0.0.1"}}
```
== Filter Config ==
```ruby
filter {
  geoip {
    source => "client_data"
    index => "ip"
    target => "geoip"
  }
}
```