Create GitHub Pages Jekyll prefetcher

[jayvdb]

GitHub Pages allows use of GitHub API via github metadata gem, and can create a json file.

It would mean 'fork' is all someone needs to do to get it working, without all the benefits of our build optimisations.
The data structure would need to be identical.
So we might want to tidy up the data structure first to eliminate some of the unnecessary duplication.
(Do we have an issue about that yet?)

[jayvdb]

We wouldnt want dist in the source tree again, so forkers might enable Github Pages on gh-pages, so we would copy the jekyll stuff on our deploy.

But then they get our old issues data in their tree. To avoid that we could have a second cleaner deploy branch for them to use.

[jayvdb]

See philschatz#57