Prepend ISO date to log lines. Fixes #2.

@rfk mind having a look? This pull request prepends the ISO date to loglines, so that deployer's logs are less insane to try to read in a pinch.

I couldn't quite figure out how to test this locally, but the code looks ok to me. Thoughts?

After the Zerigo outage ends, I can push the changes onboard deployer and verify nothing's borked.