feat(SSE support): add Server Sent Event (aka SSE) support to Gatling

- added the default support. Retry is not implemented.

see https://github.com/gatling/gatling/issues/1734

Conflicts:
	gatling-http/src/main/scala/io/gatling/http/ahc/HttpEngine.scala
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
regex constants + turn EventStreamParser into trait + FIXME
Use new JSON util
minor clean up
minor clean up + FIXME encoding
executeRequest no longer throws exceptions
FIXME
SSE use UTF-8
minor clean up
Disable SSE request timeout
Automatically set Accept header to text/event-stream
format + fix build
fix test
Add Cache-Control no-cache
Optimize parser
Merge pull request #2425 from gatling/sse

HTTP: Add SSE support