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)
Oh: the spaghetti is not gone, it’s just moved down here.

nikola.py
```py
self.config = {
…tons of settings…
    'GITHUB_SOURCE_BRANCH': 'master',
    'GITHUB_DEPLOY_BRANCH': 'gh-pages',
    'GITHUB_REMOTE_NAME': 'origin',
…even more settings…
}
```

this file
```py
        source = self.site.config['GITHUB_SOURCE_BRANCH']
        deploy = self.site.config[('GITHUB_DEPLOY_BRANCH']
        remote = self.site.config['GITHUB_REMOTE_NAME']
```