Auto-update of `guides_style_18f` gem

[mbland]

After the refactoring I'm about to do to support #20, I'll be able to add the ability to auto-update the guides_style_18f gems, or any arbitrary theme package. Initial thoughts on how I'm thinking it'll come together:

• New configuration variables:
  • defaultThemeUpdater object with repoName, updateCommand, waitDelay, and maxRetries for the top-level config
    • The delay is necessary because release tags are often pushed before their corresponding packages, but should be something reasonably small, like 10000ms
    • the default command should be bundle update --source guides_style_18f && bundle install in our case
  • optional themeUpdater object for specific builders with repo and updateCommand properties
• On startup, scan every repo for .18f.pages.yml files and find any theme_repo: and theme_update: properties
  • Skip any repo with an auto_update_theme: false property
• Create a ThemeUpdater class, which will:
  • take this list and create a mapping from theme repo names to repoName, updateCommand arrays
  • Use GitHub's PubSubHubBub API to subscribe to create events from all theme repos, so new releases can be identified by new tags
  • Update the mapping every time a site is rebuilt, as part of .18f-pages.yml processing
• When a theme update event comes in, the ThemeUpdater will:
  • wait for waitDelay milliseconds
  • launch the following operations for each repository in parallel, up to maxRetries times each:
    • grab the update lock (already implemented in SiteBuilder using file-locked-operation)
    • run the theme update command; if successful, rebuild the site
      • if unsuccessful, log the error, increment the retry count, wait and try again
    • if the rebuild is successful, do a git add && git commit -m 'Updated theme to <TAG-NAME> && git push
      • if the rebuild isn't successful, log the event and remove the repo from the ThemeUpdater mapping, as this may signal an incompatible update

[afeld]

Simpler option that may get us some or all of the way: use Gemnasium's auto-update feature.