Fixed id pamam's behavior to set column's value as document id

[sakama]

This pull-request is based on pull-request #11, and I corrected confliction of code.
I confirmed it worked fine.

id option of cuurrent version(v0.1.8) doesn't work correctly.

For example, I run with followwing config.yml,

out:
  type: elasticsearch
  id: id
  nodes:
    - {host: localhost, port: 9300}
  index: embulk
  index_type: embulk

I can get like follwing result.

{
    "_index" : "embulk",
    "_type" : "embulk",
    "_id" : "AVFWxh4LT8S5KFvme3FD",
    "_score" : 1.0,
    "_source":{
        "id":4,
        "account":11270,
        "time":"2015-01-29T11:54:36.000Z",
        "purchase":"2015-01-29T00:00:00.000Z",
        "comment":"NULL"
    }
}

But expected result is like below.
We can set document id correctly and we can search with id params via REST API.

$ curl -XGET localhost:9200/embulk/embulk/4
{
    "_index" : "embulk",
    "_type" : "embulk",
    "_id" : "4",
    "_score" : 1.0,
    "_source":{
        "id":4,
        "account":11270,
        "time":"2015-01-29T11:54:36.000Z",
        "purchase":"2015-01-29T00:00:00.000Z",
        "comment":"NULL"
    }
}

[sakama]

I merged.