fix(SSE parsing): SSE parsing did not work when the SSE elements are sent one by one
refactor(SSE support): refactor to take in account code review of the pull request https://github.com/gatling/gatling/pull/2393

- changed magic number 200 with « org.jboss.netty.handler.codec.http.HttpResponseStatus.OK »
- added empty lines at the end of files so that git stop complaining
- changed data as an Optional[String] + refactored the asJSONString() with Jackson
- moved EventStreamParser to a companion object so it's not created for every new AsyncHandler instance
- added the doc for the SSE support