Initial commit to use ghp-import in github_deploy.

The tool automatically rebases before each time it pushes.  We will need
to remove a config variable that currently makes this optional.
Remove unused imports.
Remove the GITHUB_PULL_BEFORE_COMMIT option.

ghp-import always tries to rebase before commits.
Don't set unnecessary attributes on CommandGitHubDeploy.

Minor refactor.
Set default values for github deploy config variables.
Add a note on installation of ghp-import.

[skip-ci]
Add ghp-import to requirements.txt

[skip ci]
Add versioned requirement for ghp-import.
Update the docs for github_deploy.

[skip ci]
Merge pull request #1509 from getnikola/ghp-import-deploy

Use ghp-import in the github_deploy command.