Allow to change logger formatter.

In Heroku, logging time are provided by logplex(?).
so I want to remove datetime as like `07/28/14 06:57:45 +0000` from newrelic agent logging.

```
2014-07-28T06:57:45.391666+00:00 app[web.1]: ** [NewRelic][07/28/14 06:57:45 +0000 web.1 (2)] INFO : Starting the New Relic agent in "production" environment.
2014-07-28T06:57:45.391678+00:00 app[web.1]: ** [NewRelic][07/28/14 06:57:45 +0000 web.1 (2)] INFO : To prevent agent startup add a NEWRELIC_AGENT_ENABLED=false environment variable or modify the "production" section of your newrelic.yml.
2014-07-28T06:57:45.391714+00:00 app[web.1]: ** [NewRelic][07/28/14 06:57:45 +0000 web.1 (2)] INFO : Reading configuration from config/newrelic.yml
2014-07-28T06:57:45.395774+00:00 app[web.1]: ** [NewRelic][07/28/14 06:57:45 +0000 web.1 (2)] INFO : Environment: production
```