user: return error on duplicate key insertion

Closes #47
AUTHORS, CONTRIBUTORS: added Rodrigo Chacon
Created a file to make the backup for authorized_keys.
Making the backup of repositories files.
Using s3cmd put instead cp.
Fixed the compact function.
Inital structure for docs.
Added sphinx in requirements.txt.
Added docs for install gandalf.
Added a quickstart guide in docs.
Ignoring docs/build.
Added an index for gandalf docs.
Fixed the rst formating on install docs.
Added a link for installation guide in quickstart.
Added docs about gandalf backup.
Refactored the backup.bash
Fixed the project name.
Added macking backup in the docs index.
Added docs about how to contribute with gandalf.
Fixed a typo on docs/index.