Fix CLI tests to test exit when Python < 2.6

This completes the coverage that was lost in recent changes.
Fix commandline help messages.

The help message in the commandline now gives a brief description of SALVE and
links to the website for detailed documentation. Also, disabled the backup
subcommand, so that it no longer appears as a valid subcommand in the help.
Merge branch 'documentation/cli-help' into dev