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)