Support message_template, message_key configuration

After merging this request, this plugin allows user to set up message format and keys used in the format. 

For example, 

```
message_template %s - [%s] : %s
message_key level,timestamp,message
```

and Ruby scripts says: 

```ruby
fluent_logger.post('hipchat', {
  'level' => 'INFO',
  'timestamp' => Time.now.to_s,
  'message' => 'foor'
})
```

Then this plugin send the following message to the room on HipChat. 

`INFO - [2013-11-12 00:24:24 +0900] : foo`