WIP: Version check

This is a work in progress proof of concept for potentially monkey-patching Jekyll to verify that it's running the same versions of Jekyll and related dependencies as GitHub Pages is running in production, and if an update is necessary, to provide a warning level error at build time and encouraging the user to update (allowing the build to continue).

The check hits http://pages.github.com/versions.json, and compare the `github-pages` Gem version listed to the verseion presently installed.

@parkr, @mattr- thoughts on the best way to hook into Jekyll's build process?