log to stdout xor stderr, depending on severity

[krancour]

Fixes #25

@bingosummer can you please verify that the source in this branch produces the desired results when running the broker in Cloud Foundry?

[arschles]

Thanks for this @krancour

I'm going to leave this unmerged until @bingosummer verifies it works properly on CF

[arschles]

gonna leave this on your branch if/until logrus fixes? if so, can you put some doc here with a link to the issue/PR?

[krancour]

Yes. Upstream PR isn't open just yet. I did leave a comment about this in broker.go. I can add one here as well.

[krancour]

Commented in both places.

[krancour]

Ok... I'm no CF expert, but I decided to dip my toes in that pool and try this myself. I added two new debug statements to cmd/broker/broker.go to test this:

func main() {
	log.Info("This is NOT an error!")
	log.Error("This IS an error!")
	// ... existing code ...
}

And the results in the logs are this:

$ cf logs asb --recent
...
2017-11-16T13:55:41.95-0500 [APP/PROC/WEB/0] ERR time="2017-11-16T18:55:41Z" level=error msg="This IS an error!"
   2017-11-16T13:55:41.95-0500 [APP/PROC/WEB/0] OUT time="2017-11-16T18:55:41Z" level=info msg="This is NOT an error!"

I'm satisfied that this change is working as expected vis-a-vis CF. Merging.