Start the repo over

Reboots the repo with much less content and assets, updates asset URLs to be relative, de-forks it (in GitHub) from POD. Also updates gem dependencies (e.g. `github-pages`).

There is still more work to do:

* Swap out `Project Open Data` branding for something else
* Update URLs and metadata in `_config.yml`
* Find any remaining uses of `project-open-data` or `-project-open-data.github.io` in the code and rename them to `usaid` or `usaid.github.io`.

Also, this repository is currently at `http://usaid.github.io/implementation-guide/` and not `http://usaid.github.io` because we had trouble with GitHub when we renamed the repo to `usaid.github.io`. We got the error "User must be a human, not an Organization" on the Settings page, where the GitHub Pages build information would be. Not sure what's up with that.

cc @benbalter