Use ghp-import in the github_deploy command.

This PR changes the `nikola github_deploy` to use [ghp-import](https://github.com/davisp/ghp-import) for deploying to GitHub pages. 

The tool always tries to rebase from remote, before committing.  This commit, therefore, removes the `GITHUB_PULL_BEFORE_COMMIT` config variable. 

The change was triggered by [this post](http://govlab.github.io/6109/posts/static-site-comparison-nikola-vs-pelican.html)
Please add a comment for future generations stating

```py
# req_missing defaults to `python=True` — and it’s meant to be like this.
# `ghp-import` is installed via pip, but the only way to use it is by executing the script it installs.
```

(bonus nitpick: capitalize the P in <em>GitHub <strong>P</strong>ages</em>.)