Added support for logging via the supervisor

[ssttevee]

Resolves #13.

The batching logic mirrors that of the logger from the worker.js retrieved by following the instructions from #16.

The logger may be used directly:

nodego.InfoLogger.Println("Hello World!")

or via the log package after calling nodego.OverrideLogger()

func init() {
    nodego.OverrideLogger()
}

...

log.Println("Hello World")

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[ssttevee]

I signed it!

[googlebot]

CLAs look good, thanks!

[ssttevee]

I added a WithLogger middleware function that gets the Function-Execution-Id header and includes in new log entries. I'm not sure there's anything else for it to do.

[ssttevee]

I have moved things around and un-exported the functions that don't need to be exported. I also added the flushing mechanism but somewhere in the back of my mind, it doesn't feel like the best approach.

[iangudger]

Also update the README.md file?

It still says "Logging is not supported (yet)." https://github.com/GoogleCloudPlatform/cloud-functions-go#limitations

[iangudger]

All of the new files need a copyright/license header at the top. Ex: https://github.com/GoogleCloudPlatform/cloud-functions-go/blob/master/nodego/nodego.go

[ssttevee]

Moved functions related to the mutex into an named type as per your suggestion. I also fixed some other bugs here and there. Added readme logging section and license preambles as well.