Print mostly to stderr so users can source the export line

[benley]

This makes it possible to do things like:

source <(aws-google-auth --print-creds)

... to avoid the copy-and-paste routine that would otherwise be
necessary to set the various shell variables.

[coveralls]

Coverage decreased (-0.005%) to 47.53% when pulling 50aeb0a on benley:print-stderr into d473d67 on cevoaustralia:master.

[coveralls]

Coverage decreased (-0.005%) to 47.53% when pulling 50aeb0a on benley:print-stderr into d473d67 on cevoaustralia:master.