Implement get_backup + note on create_backup

Completes previously added create_backup function. You can now
either only create a backup or create and download it.

The naming get_backup() assumes you want a fresh backup, thus always
creates a fresh backup on the controller first.

While create_backup() is making the conversation with the API,
get_backup() completes the process of getting a backup.

A backup obtained with get_backup() was tested to restore a 2.4.5
back on another 2.4.5 controller.

Added a note on create_backup about high load and fix a style error
I introduced previously.
Update the documentation section about use of the backup functions.
OK, let's stick to the other code style, fix the get_backup doc.
Merge pull request #4 from matsimon/backup-functions

Complete backup methods