Updated zabbix output plugin to allow automatic item creation with zabbix api. 

I've modified the zabbix output plugin to use the [zabbixapi](https://github.com/express42/zabbixapi) gem to auto create zabbix items.
I've also added the possibility to use zabbix sender input file to specify the "@timestamp" event value.
The zabbix_sender file is flushed every N events.

To configure this plugin, just add fields in logstash filter :
```
    grok {
        match => [ "message", "%{SYSLOGBASE} %{DATA:data}" ]
            add_tag => [ "zabbix-sender" ]
            add_field => [
                "zabbix_host", "%{source_host}",
                "zabbix_item_key", "item.key",
                "zabbix_item_name", "Item name",
                "zabbix_item_description", "Item description",
                "zabbix_item_value_type", 0,
                "zabbix_item_data_type", 0,
                "zabbix_item_unit", "record",
                "send_field", "data"
            ]
    }
```
And then configure the output like this :
```
    output {
        zabbix {
            host => "localhost"
            port => "10051"
            zabbix_sender => "/usr/local/bin/zabbix_sender"
            zabbix_url => "http://localhost/zabbix/api_jsonrpc.php"
            zabbix_user => "api"
            zabbix_pass => "password"
            zabbix_sender_host => "localhost"
            zabbix_user_sender_file => true
            zabbix_sender_file_path => "/tmp/zabbix_sender_
            zabbix_sender_file_with_timestamp => true
            zabbix_sender_flush_count => 10
        }
    }
```
More documentation of available parameters is available in the lib/logstash/outputs/zabbix.rb source file.