Added @type and @tags support

now layout contains _@type_ or _type_  (depends on layout version), using property logstashType.

I tried to integrate it with elasticsearch and it is no possible to specify _type_ property because it is already reserved.
Here is my test config
```
  rabbitmq: 
    type: com.plant42.log4j.appenders.RabbitMQAppender
    identifier: test_elasticsearch
    host: localhost
    port: 5672
    username: guest
    password: guest
    virtualHost: /
    exchange: logstash
    #type: direct  # unable to use type
    durable: true
    queue: logstash-queue
    layout:
      type: net.logstash.log4j.JSONEventLayoutV1
      tags: "es_test,rabbitmq"
      locationInfo: true
      logstashType: "elasticsearch_test"
```
Hope logstashType is good name. 
Tag - comma-separated list without space after comma.
Looks like in new version logstash no option _source__host_ but _host_ exists. So I changed it.