Add create_archives worker script

The create archives worker scripts reads in page sets generated from Alexa's
top1M CSV and captures archives. These archives are cached locally and
uploaded to Google Storage.

Also made the following improvements:
* Added ability to download multiple levels of directory in gs.DownloadWorkerArtifacts.
* Added utility to execute commands with a timeout. Using this in both create_pagesets and capture_archives.

BUG=skia:3163

Review URL: https://codereview.chromium.org/778123004